core: add experimental per-session git worktree mode

Introduce WorktreeHandle to create/reuse/remove linked checkouts under <cwd>/codex/<conversation>; add config flag enable_git_worktree (and trust logic for worktrees); plumb protocol: RemoveWorktree, WorktreeRemoved, SessionConfigured.worktree_path; update exec/TUI integration; docs: experimental config section.

# Conflicts:
#	codex-rs/core/src/codex.rs
#	codex-rs/tui/src/chatwidget.rs

codex-rs/mcp-server/src/codex_tool_runner.rs

@@ -278,6 +278,7 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::PlanUpdate(_)
                     | EventMsg::TurnAborted(_)
                     | EventMsg::ConversationPath(_)
+                    | EventMsg::WorktreeRemoved(_)
                     | EventMsg::UserMessage(_)
                     | EventMsg::ShutdownComplete
                     | EventMsg::EnteredReviewMode(_)