Protect first-time project .codex creation across Linux and macOS sandboxes (#15067)

## Problem

Codex already treated an existing top-level project `./.codex` directory
as protected, but there was a gap on first creation.

If `./.codex` did not exist yet, a turn could create files under it,
such as `./.codex/config.toml`, without going through the same approval
path as later modifications. That meant the initial write could bypass
the intended protection for project-local Codex state.

## What this changes

This PR closes that first-creation gap in the Unix enforcement layers:

- `codex-protocol`
- treat the top-level project `./.codex` path as a protected carveout
even when it does not exist yet
- avoid injecting the default carveout when the user already has an
explicit rule for that exact path
- macOS Seatbelt
- deny writes to both the exact protected path and anything beneath it,
so creating `./.codex` itself is blocked in addition to writes inside it
- Linux bubblewrap
- preserve the same protected-path behavior for first-time creation
under `./.codex`
- tests
- add protocol regressions for missing `./.codex` and explicit-rule
collisions
- add Unix sandbox coverage for blocking first-time `./.codex` creation
  - tighten Seatbelt policy assertions around excluded subpaths

## Scope

This change is intentionally scoped to protecting the top-level project
`.codex` subtree from agent writes.

It does not make `.codex` unreadable, and it does not change the product
behavior around loading project skills from `.codex` when project config
is untrusted.

## Why this shape

The fix is pointed rather than broad:
- it preserves the current model of “project `.codex` is protected from
writes”
- it closes the security-relevant first-write hole
- it avoids folding a larger permissions-model redesign into this PR

## Validation

- `cargo test -p codex-protocol`
- `cargo test -p codex-sandboxing seatbelt`
- `cargo test -p codex-exec --test all
sandbox_blocks_first_time_dot_codex_creation -- --nocapture`

---------

Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/core/src/exec_tests.rs

@@ -595,6 +595,19 @@ fn windows_restricted_token_supports_full_read_split_write_read_carveouts() {
         },
     ]);
 
+    #[cfg(windows)]
+    let expected_deny_write_paths = vec![
+        codex_utils_absolute_path::AbsolutePathBuf::from_absolute_path(cwd.join(".codex"))
+            .expect("absolute .codex"),
+        codex_utils_absolute_path::AbsolutePathBuf::from_absolute_path(&docs)
+            .expect("absolute docs"),
+    ];
+    #[cfg(not(windows))]
+    let expected_deny_write_paths = vec![
+        codex_utils_absolute_path::AbsolutePathBuf::from_absolute_path(&docs)
+            .expect("absolute docs"),
+    ];
+
     assert_eq!(
         resolve_windows_restricted_token_filesystem_overlay(
             SandboxType::WindowsRestrictedToken,
@@ -605,10 +618,7 @@ fn windows_restricted_token_supports_full_read_split_write_read_carveouts() {
             WindowsSandboxLevel::RestrictedToken,
         ),
         Ok(Some(WindowsRestrictedTokenFilesystemOverlay {
-            additional_deny_write_paths: vec![
-                codex_utils_absolute_path::AbsolutePathBuf::from_absolute_path(&docs)
-                    .expect("absolute docs"),
-            ],
+            additional_deny_write_paths: expected_deny_write_paths,
         }))
     );
 }