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/core/src/session/session.rs

@@ -121,7 +121,7 @@ impl SessionConfiguration {
     pub(crate) fn apply(&self, updates: &SessionSettingsUpdate) -> ConstraintResult<Self> {
         let mut next_configuration = self.clone();
         let file_system_policy_matches_legacy = self.file_system_sandbox_policy
-            == FileSystemSandboxPolicy::from_legacy_sandbox_policy(
+            == FileSystemSandboxPolicy::from_legacy_sandbox_policy_for_cwd(
                 self.sandbox_policy.get(),
                 &self.cwd,
             );
@@ -201,7 +201,7 @@ impl SessionConfiguration {
             // Preserve richer split policies across cwd-only updates; only
             // rederive when the session is already using the legacy bridge.
             next_configuration.file_system_sandbox_policy =
-                FileSystemSandboxPolicy::from_legacy_sandbox_policy(
+                FileSystemSandboxPolicy::from_legacy_sandbox_policy_for_cwd(
                     next_configuration.sandbox_policy.get(),
                     &next_configuration.cwd,
                 );