feat: introduce Permissions (#11633)

## Why
We currently carry multiple permission-related concepts directly on
`Config` for shell/unified-exec behavior (`approval_policy`,
`sandbox_policy`, `network`, `shell_environment_policy`,
`windows_sandbox_mode`).

Consolidating these into one in-memory struct makes permission handling
easier to reason about and sets up the next step: supporting named
permission profiles (`[permissions.PROFILE_NAME]`) without changing
behavior now.

This change is mostly mechanical: it updates existing callsites to go
through `config.permissions`, but it does not yet refactor those
callsites to take a single `Permissions` value in places where multiple
permission fields are still threaded separately.

This PR intentionally **does not** change the on-disk `config.toml`
format yet and keeps compatibility with legacy config keys.

## What Changed
- Introduced `Permissions` in `core/src/config/mod.rs`.
- Added `Config::permissions` and moved effective runtime permission
fields under it:
  - `approval_policy`
  - `sandbox_policy`
  - `network`
  - `shell_environment_policy`
  - `windows_sandbox_mode`
- Updated config loading/building so these effective values are still
derived from the same existing config inputs and constraints.
- Updated Windows sandbox helpers/resolution to read/write via
`permissions`.
- Threaded the new field through all permission consumers across core
runtime, app-server, CLI/exec, TUI, and sandbox summary code.
- Updated affected tests to reference `config.permissions.*`.
- Renamed the struct/field from
`EffectivePermissions`/`effective_permissions` to
`Permissions`/`permissions` and aligned variable naming accordingly.

## Verification
- `just fix -p codex-core -p codex-tui -p codex-cli -p codex-app-server
-p codex-exec -p codex-utils-sandbox-summary`
- `cargo build -p codex-core -p codex-tui -p codex-cli -p
codex-app-server -p codex-exec -p codex-utils-sandbox-summary`

codex-rs/app-server/src/codex_message_processor.rs

@@ -1677,11 +1677,11 @@ impl CodexMessageProcessor {
         }
 
         let cwd = params.cwd.unwrap_or_else(|| self.config.cwd.clone());
-        let env = create_env(&self.config.shell_environment_policy, None);
+        let env = create_env(&self.config.permissions.shell_environment_policy, None);
         let timeout_ms = params
             .timeout_ms
             .and_then(|timeout_ms| u64::try_from(timeout_ms).ok());
-        let started_network_proxy = match self.config.network.as_ref() {
+        let started_network_proxy = match self.config.permissions.network.as_ref() {
             Some(spec) => match spec.start_proxy().await {
                 Ok(started) => Some(started),
                 Err(err) => {
@@ -1713,7 +1713,7 @@ impl CodexMessageProcessor {
 
         let requested_policy = params.sandbox_policy.map(|policy| policy.to_core());
         let effective_policy = match requested_policy {
-            Some(policy) => match self.config.sandbox_policy.can_set(&policy) {
+            Some(policy) => match self.config.permissions.sandbox_policy.can_set(&policy) {
                 Ok(()) => policy,
                 Err(err) => {
                     let error = JSONRPCErrorError {
@@ -1725,7 +1725,7 @@ impl CodexMessageProcessor {
                     return;
                 }
             },
-            None => self.config.sandbox_policy.get().clone(),
+            None => self.config.permissions.sandbox_policy.get().clone(),
         };
 
         let codex_linux_sandbox_exe = self.config.codex_linux_sandbox_exe.clone();