Add worktree settings parser (#40624)

## What changed

- Add the `codex-worktree` crate for resolving managed worktree settings from
  the existing `[desktop]` configuration.
- Default the worktree root to `$CODEX_HOME/worktrees`, automatic cleanup to
  enabled, and retained worktrees to 15.
- Validate configured roots, cleanup flags, and retention counts before
  exposing the effective settings.

## Testing

- Cover defaults, configured values, and invalid root and retention settings.

GitOrigin-RevId: 1b279c62ef41e3c8eb540ce9beaf0f6cd96d903f

codex-rs/worktree/BUILD.bazel

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "worktree",
+    crate_name = "codex_worktree",
+)