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/worktree/src/git_tests.rs

@@ -166,3 +166,33 @@ fn write_marker_script(path: &Path, marker: &Path) -> Result<()> {
     fs::set_permissions(path, fs::Permissions::from_mode(0o755))?;
     Ok(())
 }
+
+#[cfg(unix)]
+#[test]
+fn default_base_ignores_unrelated_non_utf8_refs() -> Result<()> {
+    let (_directory, root) = repository()?;
+    let head = git_stdout(&root, ["rev-parse", "HEAD"])?;
+    // Packed refs work even on filesystems that cannot store non-UTF-8 filenames.
+    fs::write(
+        root.join(".git/packed-refs"),
+        [head.as_bytes(), b" refs/heads/unrelated-\xff\n"].concat(),
+    )?;
+    assert_eq!(super::default_worktree_base(&root)?, "refs/heads/main");
+    setup_git(
+        &root,
+        &["update-ref", "refs/remotes/origin/release/stable", "HEAD"],
+    )?;
+    setup_git(
+        &root,
+        &[
+            "symbolic-ref",
+            "refs/remotes/origin/HEAD",
+            "refs/remotes/origin/release/stable",
+        ],
+    )?;
+    assert_eq!(
+        super::default_worktree_base(&root)?,
+        "refs/remotes/origin/release/stable"
+    );
+    Ok(())
+}