Add worktree session creation to the agents overview (#45276)

## What changed

- Add a configurable `new_worktree` action, bound to `w`, for local sessions with worktree support enabled.
- Create the worktree from the cached project default branch, preferring remote `HEAD` and falling back to conventional `main` or `master` refs without fetching. Preserve the selected subdirectory and source checkout edits.
- Start and bind a blank session in the new worktree, preserving the selected approval reviewer without sending an initial turn.
- Show creation progress, pause conflicting actions, clean up unclaimed clean worktrees, and report retained checkout paths when session startup fails. Clear pending creation state on reconnect.

## Testing

Add coverage for default-branch selection, source checkout preservation, session ownership and approval settings, cleanup, startup failures, reconnect state, and busy-state rendering. Extend Git environment isolation coverage and test unrelated non-UTF-8 refs.

GitOrigin-RevId: 771ddc12d04dbf0d12e77e9c96c8386cf31e67c5

codex-rs/core/config.schema.json

@@ -4090,6 +4090,7 @@
               "delete": null,
               "hide": null,
               "new_task": null,
+              "new_worktree": null,
               "rename": null,
               "resume": null,
               "search": null,
@@ -4431,6 +4432,14 @@
           ],
           "description": "Open a new session in the selected checkout."
         },
+        "new_worktree": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/KeybindingsSpec"
+            }
+          ],
+          "description": "Open a new session in a worktree from the project default branch."
+        },
         "rename": {
           "allOf": [
             {
@@ -4932,6 +4941,7 @@
             "delete": null,
             "hide": null,
             "new_task": null,
+            "new_worktree": null,
             "rename": null,
             "resume": null,
             "search": null,