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/models.rs

@@ -429,10 +429,10 @@ impl PermissionProfile {
         }
     }
 
-    pub fn from_legacy_sandbox_policy(sandbox_policy: &SandboxPolicy, cwd: &Path) -> Self {
+    pub fn from_legacy_sandbox_policy(sandbox_policy: &SandboxPolicy) -> Self {
         Self::from_runtime_permissions_with_enforcement(
             SandboxEnforcement::from_legacy_sandbox_policy(sandbox_policy),
-            &FileSystemSandboxPolicy::from_legacy_sandbox_policy(sandbox_policy, cwd),
+            &FileSystemSandboxPolicy::from_legacy_sandbox_policy(sandbox_policy),
             NetworkSandboxPolicy::from(sandbox_policy),
         )
     }
@@ -1765,10 +1765,8 @@ mod tests {
     #[test]
     fn permission_profile_round_trip_preserves_disabled_sandbox() -> Result<()> {
         let cwd = tempdir()?;
-        let permission_profile = PermissionProfile::from_legacy_sandbox_policy(
-            &SandboxPolicy::DangerFullAccess,
-            cwd.path(),
-        );
+        let permission_profile =
+            PermissionProfile::from_legacy_sandbox_policy(&SandboxPolicy::DangerFullAccess);
 
         assert_eq!(permission_profile, PermissionProfile::Disabled);
         assert_eq!(
@@ -1839,8 +1837,7 @@ mod tests {
         let sandbox_policy = SandboxPolicy::ExternalSandbox {
             network_access: crate::protocol::NetworkAccess::Restricted,
         };
-        let permission_profile =
-            PermissionProfile::from_legacy_sandbox_policy(&sandbox_policy, cwd.path());
+        let permission_profile = PermissionProfile::from_legacy_sandbox_policy(&sandbox_policy);
 
         assert_eq!(
             permission_profile,