permissions: derive config defaults as profiles (#19772)

## Why

This continues the permissions migration by making legacy config default
resolution produce the canonical `PermissionProfile` first. The legacy
`SandboxPolicy` projection should stay available at compatibility
boundaries, but config loading should not create a legacy policy just to
immediately convert it back into a profile.

Specifically, when `default_permissions` is not specified in
`config.toml`, instead of creating a `SandboxPolicy` in
`codex-rs/core/src/config/mod.rs` and then trying to derive a
`PermissionProfile` from it, we use `derive_permission_profile()` to
create a more faithful `PermissionProfile` using the values of
`ConfigToml` directly.

This also keeps the existing behavior of `sandbox_workspace_write` and
extra writable roots after #19841 replaced `:cwd` with `:project_roots`.
Legacy workspace-write defaults are represented as symbolic
`:project_roots` write access plus symbolic project-root metadata
carveouts. Extra absolute writable roots are still added directly and
continue to get concrete metadata protections for paths that exist under
those roots.

The platform sandboxes differ when a symbolic project-root subpath does
not exist yet.

* **Seatbelt** can encode literal/subpath exclusions directly, so macOS
emits project-root metadata subpath policies even if `.git`, `.agents`,
or `.codex` do not exist.
* **bwrap** has to materialize bind-mount targets. Binding `/dev/null`
to a missing `.git` can create a host-visible placeholder that changes
Git repo discovery. Binding missing `.agents` would not affect Git
discovery, but it would still create a host-visible project metadata
placeholder from an automatic compatibility carveout. Linux therefore
skips only missing automatic `.git` and `.agents` read-only metadata
masks; missing `.codex` remains protected so first-time project config
creation goes through the protected-path approval flow. User-authored
`read` and `none` subpath rules keep normal bwrap behavior, and `none`
can still mask the first missing component to prevent creation under
writable roots.

## What Changed

- Adds profile-native helpers for legacy workspace-write semantics,
including `PermissionProfile::workspace_write_with()`,
`FileSystemSandboxPolicy::workspace_write()`, and
`FileSystemSandboxPolicy::with_additional_legacy_workspace_writable_roots()`.
- Makes `FileSystemSandboxPolicy::workspace_write()` the single legacy
workspace-write constructor so both `from_legacy_sandbox_policy()` and
`From<&SandboxPolicy>` include the project-root metadata carveouts.
- Removes the no-carveout `legacy_workspace_write_base_policy()` path
and the `prune_read_entries_under_writable_roots()` cleanup that was
only needed by that split construction.
- Adds `ConfigToml::derive_permission_profile()` for legacy sandbox-mode
fallback resolution; named `default_permissions` profiles continue
through the permissions profile pipeline instead of being reconstructed
from `sandbox_mode`.
- Updates `Config::load()` to start from the derived profile, validate
that it still has a legacy compatibility projection, and apply
additional writable roots directly to managed workspace-write filesystem
policies.
- Updates Linux bwrap argument construction so missing automatic
`.git`/`.agents` symbolic project-root read-only carveouts are skipped
before emitting bind args; missing `.codex`, user-authored `read`/`none`
subpath rules, and existing missing writable-root behavior are
preserved.
- Adds coverage that legacy workspace-write config produces symbolic
project-root metadata carveouts, extra legacy workspace writable roots
still protect existing metadata paths such as `.git`, and bwrap skips
missing `.git`/`.agents` project-root carveouts while preserving missing
`.codex` and user-authored missing subpath rules.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/19772).
* #19776
* #19775
* #19774
* #19773
* __->__ #19772

codex-rs/config/src/config_toml.rs

@@ -49,8 +49,8 @@ use codex_protocol::config_types::WebSearchToolConfig;
 use codex_protocol::config_types::WindowsSandboxLevel;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::openai_models::ReasoningEffort;
+use codex_protocol::permissions::NetworkSandboxPolicy;
 use codex_protocol::protocol::AskForApproval;
-use codex_protocol::protocol::SandboxPolicy;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_path::normalize_for_path_comparison;
 use schemars::JsonSchema;
@@ -641,15 +641,19 @@ pub struct GhostSnapshotToml {
 }
 
 impl ConfigToml {
-    /// Derive the effective sandbox policy from the configuration.
-    pub async fn derive_sandbox_policy(
+    /// Derive the effective permission profile from legacy sandbox config.
+    ///
+    /// Call this only after ruling out `default_permissions`: named
+    /// `[permissions]` profiles must be compiled through the permissions
+    /// profile pipeline, not reconstructed from `sandbox_mode`.
+    pub async fn derive_permission_profile(
         &self,
         sandbox_mode_override: Option<SandboxMode>,
         profile_sandbox_mode: Option<SandboxMode>,
         windows_sandbox_level: WindowsSandboxLevel,
         active_project: Option<&ProjectConfig>,
         permission_profile_constraint: Option<&crate::Constrained<PermissionProfile>>,
-    ) -> SandboxPolicy {
+    ) -> PermissionProfile {
         let sandbox_mode_was_explicit = sandbox_mode_override.is_some()
             || profile_sandbox_mode.is_some()
             || self.sandbox_mode.is_some();
@@ -677,50 +681,53 @@ impl ConfigToml {
                 })
             })
             .unwrap_or_default();
-        let mut sandbox_policy = match resolved_sandbox_mode {
-            SandboxMode::ReadOnly => SandboxPolicy::new_read_only_policy(),
+        let effective_sandbox_mode = if cfg!(target_os = "windows")
+            // If the experimental Windows sandbox is enabled, do not force a downgrade.
+            && windows_sandbox_level == WindowsSandboxLevel::Disabled
+            && matches!(resolved_sandbox_mode, SandboxMode::WorkspaceWrite)
+        {
+            SandboxMode::ReadOnly
+        } else {
+            resolved_sandbox_mode
+        };
+
+        let permission_profile = match effective_sandbox_mode {
+            SandboxMode::ReadOnly => PermissionProfile::read_only(),
             SandboxMode::WorkspaceWrite => match self.sandbox_workspace_write.as_ref() {
                 Some(SandboxWorkspaceWrite {
                     writable_roots,
                     network_access,
                     exclude_tmpdir_env_var,
                     exclude_slash_tmp,
-                }) => SandboxPolicy::WorkspaceWrite {
-                    writable_roots: writable_roots.clone(),
-                    network_access: *network_access,
-                    exclude_tmpdir_env_var: *exclude_tmpdir_env_var,
-                    exclude_slash_tmp: *exclude_slash_tmp,
-                },
-                None => SandboxPolicy::new_workspace_write_policy(),
+                }) => {
+                    let network_policy = if *network_access {
+                        NetworkSandboxPolicy::Enabled
+                    } else {
+                        NetworkSandboxPolicy::Restricted
+                    };
+                    PermissionProfile::workspace_write_with(
+                        writable_roots,
+                        network_policy,
+                        *exclude_tmpdir_env_var,
+                        *exclude_slash_tmp,
+                    )
+                }
+                None => PermissionProfile::workspace_write(),
             },
-            SandboxMode::DangerFullAccess => SandboxPolicy::DangerFullAccess,
+            SandboxMode::DangerFullAccess => PermissionProfile::Disabled,
         };
-        let downgrade_workspace_write_if_unsupported = |policy: &mut SandboxPolicy| {
-            if cfg!(target_os = "windows")
-                // If the experimental Windows sandbox is enabled, do not force a downgrade.
-                && windows_sandbox_level == WindowsSandboxLevel::Disabled
-                && matches!(&*policy, SandboxPolicy::WorkspaceWrite { .. })
-            {
-                *policy = SandboxPolicy::new_read_only_policy();
-            }
-        };
-        if matches!(resolved_sandbox_mode, SandboxMode::WorkspaceWrite) {
-            downgrade_workspace_write_if_unsupported(&mut sandbox_policy);
-        }
         if !sandbox_mode_was_explicit
             && let Some(constraint) = permission_profile_constraint
-            && let Err(err) = constraint.can_set(&PermissionProfile::from_legacy_sandbox_policy(
-                &sandbox_policy,
-            ))
+            && let Err(err) = constraint.can_set(&permission_profile)
         {
             tracing::warn!(
                 error = %err,
                 "default sandbox policy is disallowed by requirements; falling back to required default"
             );
-            sandbox_policy = SandboxPolicy::new_read_only_policy();
-            downgrade_workspace_write_if_unsupported(&mut sandbox_policy);
+            PermissionProfile::read_only()
+        } else {
+            permission_profile
         }
-        sandbox_policy
     }
 
     /// Resolves the cwd to an existing project, or returns None if ConfigToml