permissions: make legacy profile conversion cwd-free (#19414)

## Why

The profile conversion path still required a `cwd` even when it was only
translating a legacy `SandboxPolicy` into a `PermissionProfile`. That
made profile producers invent an ambient `cwd`, which is exactly the
anchoring we are trying to remove from permission-profile data. A legacy
workspace-write policy can be represented symbolically instead: `:cwd =
write` plus read-only `:project_roots` metadata subpaths.

This PR creates that cwd-free base so the rest of the stack can stop
threading cwd through profile construction. Callers that actually need a
concrete runtime filesystem policy for a specific cwd still have an
explicitly named cwd-bound conversion.

## What Changed

- `PermissionProfile::from_legacy_sandbox_policy` now takes only
`&SandboxPolicy`.
- `FileSystemSandboxPolicy::from_legacy_sandbox_policy` is now the
symbolic, cwd-free projection for profiles.
- The old concrete projection is retained as
`FileSystemSandboxPolicy::from_legacy_sandbox_policy_for_cwd` for
runtime/boundary code that must materialize legacy cwd behavior.
- Workspace-write profiles preserve `CurrentWorkingDirectory` and
`ProjectRoots` special entries instead of materializing cwd into
absolute paths.

## Verification

- `cargo check -p codex-protocol -p codex-core -p
codex-app-server-protocol -p codex-app-server -p codex-exec -p
codex-exec-server -p codex-tui -p codex-sandboxing -p
codex-linux-sandbox -p codex-analytics --tests`
- `just fix -p codex-protocol -p codex-core -p codex-app-server-protocol
-p codex-app-server -p codex-exec -p codex-exec-server -p codex-tui -p
codex-sandboxing -p codex-linux-sandbox -p codex-analytics`




---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/19414).
* #19395
* #19394
* #19393
* #19392
* #19391
* __->__ #19414

codex-rs/protocol/src/protocol.rs

@@ -3058,7 +3058,7 @@ impl TurnContextItem {
         self.permission_profile.clone().unwrap_or_else(|| {
             let file_system_sandbox_policy =
                 self.file_system_sandbox_policy.clone().unwrap_or_else(|| {
-                    FileSystemSandboxPolicy::from_legacy_sandbox_policy(
+                    FileSystemSandboxPolicy::from_legacy_sandbox_policy_for_cwd(
                         &self.sandbox_policy,
                         &self.cwd,
                     )
@@ -4644,7 +4644,7 @@ mod tests {
 
         assert_eq!(
             sorted_writable_roots(
-                FileSystemSandboxPolicy::from_legacy_sandbox_policy(&policy, cwd.path())
+                FileSystemSandboxPolicy::from_legacy_sandbox_policy_for_cwd(&policy, cwd.path())
                     .get_writable_roots_with_cwd(cwd.path())
             ),
             vec![(canonical_cwd, vec![expected_dot_codex.to_path_buf()])]
@@ -4736,9 +4736,10 @@ mod tests {
         ];
 
         for expected in policies {
-            let actual = FileSystemSandboxPolicy::from_legacy_sandbox_policy(&expected, cwd.path())
-                .to_legacy_sandbox_policy(NetworkSandboxPolicy::from(&expected), cwd.path())
-                .expect("legacy bridge should preserve legacy policy semantics");
+            let actual =
+                FileSystemSandboxPolicy::from_legacy_sandbox_policy_for_cwd(&expected, cwd.path())
+                    .to_legacy_sandbox_policy(NetworkSandboxPolicy::from(&expected), cwd.path())
+                    .expect("legacy bridge should preserve legacy policy semantics");
 
             assert_same_sandbox_policy_semantics(&expected, &actual, cwd.path());
         }