permissions: remove cwd special path (#19841)

## Why

The experimental `PermissionProfile` API had both `:cwd` and
`:project_roots` special filesystem paths, which made the permission
root ambiguous. This PR removes the unstable `current_working_directory`
special path before the permissions API is stabilized, so callers use
`:project_roots` for symbolic project-root access.

## What changed

- Removes `FileSystemSpecialPath::CurrentWorkingDirectory` from protocol
and app-server protocol models, plus regenerated app-server
JSON/TypeScript schemas.
- Replaces internal `:cwd` permission entries with `:project_roots`
entries.
- Keeps the existing cwd-update behavior for legacy-shaped
workspace-write profiles, while removing the deleted
`CurrentWorkingDirectory` case from that compatibility path.
- Keeps `PermissionProfile::workspace_write()` as the reusable symbolic
workspace-write helper, with docs noting that `:project_roots` entries
resolve at enforcement time.
- Updates app-server docs/examples and approval UI labeling to stop
advertising `:cwd` as a permission token.

## Compatibility

Persisted rollout items may contain the old
`{"kind":"current_working_directory"}` tag from earlier experimental
`permissionProfile` snapshots. This PR keeps that tag as a
deserialize-only alias for `ProjectRoots { subpath: None }`, while
continuing to serialize only the new `project_roots` tag.

## Follow-up

This PR intentionally does not introduce an explicit project-root set on
`SessionConfiguration` or runtime sandbox resolution. Today, the
resolver still uses the active cwd as the single implicit project root.
A follow-up should model project roots separately from tool cwd so
`:project_roots` entries can resolve against the configured project
roots, and resolve to no entries when there are no project roots.

## Verification

- `cargo test -p codex-protocol permissions:: --lib`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-sandboxing -p codex-exec-server --lib`
- `cargo test -p codex-core session_configuration_apply_ --lib`
- `cargo test -p codex-app-server
command_exec_permission_profile_project_roots_use_command_cwd --test
all`
- `cargo test -p codex-tui
thread_read_session_state_does_not_reuse_primary_permission_profile
--lib`
- `cargo test -p codex-tui
preset_matching_accepts_workspace_write_with_extra_roots --lib`
- `cargo test -p codex-config --lib`

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -1028,21 +1028,6 @@
           "title": "MinimalFileSystemSpecialPath",
           "type": "object"
         },
-        {
-          "properties": {
-            "kind": {
-              "enum": [
-                "current_working_directory"
-              ],
-              "type": "string"
-            }
-          },
-          "required": [
-            "kind"
-          ],
-          "title": "CurrentWorkingDirectoryFileSystemSpecialPath",
-          "type": "object"
-        },
         {
           "properties": {
             "kind": {
@@ -6124,4 +6109,4 @@
     }
   ],
   "title": "ClientRequest"
-}
+}