fix: canonicalize symlinked Linux sandbox cwd (#14849)

## Problem
On Linux, Codex can be launched from a workspace path that is a symlink
(for example, a symlinked checkout or a symlinked parent directory).

Our sandbox policy intentionally canonicalizes writable/readable roots
to the real filesystem path before building the bubblewrap mounts. That
part is correct and needed for safety.

The remaining bug was that bubblewrap could still inherit the helper
process's logical cwd, which might be the symlinked alias instead of the
mounted canonical path. In that case, the sandbox starts in a cwd that
does not exist inside the sandbox namespace even though the real
workspace is mounted. This can cause sandboxed commands to fail in
symlinked workspaces.

## Fix
This PR keeps the sandbox policy behavior the same, but separates two
concepts that were previously conflated:

- the canonical cwd used to define sandbox mounts and permissions
- the caller's logical cwd used when launching the command

On the Linux bubblewrap path, we now thread the logical command cwd
through the helper explicitly and only add `--chdir <canonical path>`
when the logical cwd differs from the mounted canonical path.

That means:
- permissions are still computed from canonical paths
- bubblewrap starts the command from a cwd that definitely exists inside
the sandbox
- we do not widen filesystem access or undo the earlier symlink
hardening

## Why This Is Safe
This is a narrow Linux-only launch fix, not a policy change.

- Writable/readable root canonicalization stays intact.
- Protected metadata carveouts still operate on canonical roots.
- We only override bubblewrap's inherited cwd when the logical path
would otherwise point at a symlink alias that is not mounted in the
sandbox.

## Tests
- kept the existing protocol/core regression coverage for symlink
canonicalization
- added regression coverage for symlinked cwd handling in the Linux
bubblewrap builder/helper path

Local validation:
- `just fmt`
- `cargo test -p codex-protocol`
- `cargo test -p codex-core
normalize_additional_permissions_canonicalizes_symlinked_write_paths`
- `cargo clippy -p codex-linux-sandbox -p codex-protocol -p codex-core
--tests -- -D warnings`
- `cargo build --bin codex`

## Context
This is related to #14694. The earlier writable-root symlink fix
addressed the mount/permission side; this PR fixes the remaining
symlinked-cwd launch mismatch in the Linux sandbox path.

codex-rs/linux-sandbox/src/linux_run_main.rs

@@ -31,6 +31,15 @@ pub struct LandlockCommand {
     #[arg(long = "sandbox-policy-cwd")]
     pub sandbox_policy_cwd: PathBuf,
 
+    /// The logical working directory for the command being sandboxed.
+    ///
+    /// This can intentionally differ from `sandbox_policy_cwd` when the
+    /// command runs from a symlinked alias of the policy workspace. Keep it
+    /// explicit so bubblewrap can preserve the caller's logical cwd when that
+    /// alias would otherwise disappear inside the sandbox namespace.
+    #[arg(long = "command-cwd", hide = true)]
+    pub command_cwd: Option<PathBuf>,
+
     /// Legacy compatibility policy.
     ///
     /// Newer callers pass split filesystem/network policies as well so the
@@ -91,6 +100,7 @@ pub struct LandlockCommand {
 pub fn run_main() -> ! {
     let LandlockCommand {
         sandbox_policy_cwd,
+        command_cwd,
         sandbox_policy,
         file_system_sandbox_policy,
         network_sandbox_policy,
@@ -177,6 +187,7 @@ pub fn run_main() -> ! {
             };
         let inner = build_inner_seccomp_command(InnerSeccompCommandArgs {
             sandbox_policy_cwd: &sandbox_policy_cwd,
+            command_cwd: command_cwd.as_deref(),
             sandbox_policy: &sandbox_policy,
             file_system_sandbox_policy: &file_system_sandbox_policy,
             network_sandbox_policy,
@@ -186,6 +197,7 @@ pub fn run_main() -> ! {
         });
         run_bwrap_with_proc_fallback(
             &sandbox_policy_cwd,
+            command_cwd.as_deref(),
             &file_system_sandbox_policy,
             network_sandbox_policy,
             inner,
@@ -387,6 +399,7 @@ fn ensure_legacy_landlock_mode_supports_policy(
 
 fn run_bwrap_with_proc_fallback(
     sandbox_policy_cwd: &Path,
+    command_cwd: Option<&Path>,
     file_system_sandbox_policy: &FileSystemSandboxPolicy,
     network_sandbox_policy: NetworkSandboxPolicy,
     inner: Vec<String>,
@@ -395,10 +408,12 @@ fn run_bwrap_with_proc_fallback(
 ) -> ! {
     let network_mode = bwrap_network_mode(network_sandbox_policy, allow_network_for_proxy);
     let mut mount_proc = mount_proc;
+    let command_cwd = command_cwd.unwrap_or(sandbox_policy_cwd);
 
     if mount_proc
         && !preflight_proc_mount_support(
             sandbox_policy_cwd,
+            command_cwd,
             file_system_sandbox_policy,
             network_mode,
         )
@@ -416,6 +431,7 @@ fn run_bwrap_with_proc_fallback(
         inner,
         file_system_sandbox_policy,
         sandbox_policy_cwd,
+        command_cwd,
         options,
     );
     exec_vendored_bwrap(bwrap_args.args, bwrap_args.preserved_files);
@@ -438,12 +454,14 @@ fn build_bwrap_argv(
     inner: Vec<String>,
     file_system_sandbox_policy: &FileSystemSandboxPolicy,
     sandbox_policy_cwd: &Path,
+    command_cwd: &Path,
     options: BwrapOptions,
 ) -> crate::bwrap::BwrapArgs {
     let mut bwrap_args = create_bwrap_command_args(
         inner,
         file_system_sandbox_policy,
         sandbox_policy_cwd,
+        command_cwd,
         options,
     )
     .unwrap_or_else(|err| panic!("error building bubblewrap command: {err:?}"));
@@ -468,17 +486,23 @@ fn build_bwrap_argv(
 
 fn preflight_proc_mount_support(
     sandbox_policy_cwd: &Path,
+    command_cwd: &Path,
     file_system_sandbox_policy: &FileSystemSandboxPolicy,
     network_mode: BwrapNetworkMode,
 ) -> bool {
-    let preflight_argv =
-        build_preflight_bwrap_argv(sandbox_policy_cwd, file_system_sandbox_policy, network_mode);
+    let preflight_argv = build_preflight_bwrap_argv(
+        sandbox_policy_cwd,
+        command_cwd,
+        file_system_sandbox_policy,
+        network_mode,
+    );
     let stderr = run_bwrap_in_child_capture_stderr(preflight_argv);
     !is_proc_mount_failure(stderr.as_str())
 }
 
 fn build_preflight_bwrap_argv(
     sandbox_policy_cwd: &Path,
+    command_cwd: &Path,
     file_system_sandbox_policy: &FileSystemSandboxPolicy,
     network_mode: BwrapNetworkMode,
 ) -> crate::bwrap::BwrapArgs {
@@ -487,6 +511,7 @@ fn build_preflight_bwrap_argv(
         preflight_command,
         file_system_sandbox_policy,
         sandbox_policy_cwd,
+        command_cwd,
         BwrapOptions {
             mount_proc: true,
             network_mode,
@@ -591,6 +616,7 @@ fn is_proc_mount_failure(stderr: &str) -> bool {
 
 struct InnerSeccompCommandArgs<'a> {
     sandbox_policy_cwd: &'a Path,
+    command_cwd: Option<&'a Path>,
     sandbox_policy: &'a SandboxPolicy,
     file_system_sandbox_policy: &'a FileSystemSandboxPolicy,
     network_sandbox_policy: NetworkSandboxPolicy,
@@ -603,6 +629,7 @@ struct InnerSeccompCommandArgs<'a> {
 fn build_inner_seccomp_command(args: InnerSeccompCommandArgs<'_>) -> Vec<String> {
     let InnerSeccompCommandArgs {
         sandbox_policy_cwd,
+        command_cwd,
         sandbox_policy,
         file_system_sandbox_policy,
         network_sandbox_policy,
@@ -631,6 +658,12 @@ fn build_inner_seccomp_command(args: InnerSeccompCommandArgs<'_>) -> Vec<String>
         current_exe.to_string_lossy().to_string(),
         "--sandbox-policy-cwd".to_string(),
         sandbox_policy_cwd.to_string_lossy().to_string(),
+    ];
+    if let Some(command_cwd) = command_cwd {
+        inner.push("--command-cwd".to_string());
+        inner.push(command_cwd.to_string_lossy().to_string());
+    }
+    inner.extend([
         "--sandbox-policy".to_string(),
         policy_json,
         "--file-system-sandbox-policy".to_string(),
@@ -638,7 +671,7 @@ fn build_inner_seccomp_command(args: InnerSeccompCommandArgs<'_>) -> Vec<String>
         "--network-sandbox-policy".to_string(),
         network_policy_json,
         "--apply-seccomp-then-exec".to_string(),
-    ];
+    ]);
     if allow_network_for_proxy {
         inner.push("--allow-network-for-proxy".to_string());
         let proxy_route_spec = proxy_route_spec