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/tools/handlers/multi_agents_tests.rs

@@ -2101,7 +2101,7 @@ async fn spawn_agent_reapplies_runtime_sandbox_after_role_config() {
         turn.config.permissions.sandbox_policy.get().clone(),
     );
     let expected_file_system_sandbox_policy =
-        FileSystemSandboxPolicy::from_legacy_sandbox_policy(&expected_sandbox, &turn.cwd);
+        FileSystemSandboxPolicy::from_legacy_sandbox_policy_for_cwd(&expected_sandbox, &turn.cwd);
     let expected_network_sandbox_policy = NetworkSandboxPolicy::from(&expected_sandbox);
     turn.approval_policy
         .set(AskForApproval::OnRequest)
@@ -3620,7 +3620,7 @@ async fn build_agent_spawn_config_uses_turn_context_values() {
         turn.config.permissions.sandbox_policy.get().clone(),
     );
     let file_system_sandbox_policy =
-        FileSystemSandboxPolicy::from_legacy_sandbox_policy(&sandbox_policy, &turn.cwd);
+        FileSystemSandboxPolicy::from_legacy_sandbox_policy_for_cwd(&sandbox_policy, &turn.cwd);
     let network_sandbox_policy = NetworkSandboxPolicy::from(&sandbox_policy);
     turn.sandbox_policy
         .set(sandbox_policy)