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_message_processor.rs

@@ -1273,6 +1273,7 @@ fn derive_config_from_params(
         include_view_image_tool: None,
         show_raw_agent_reasoning: None,
         tools_web_search_request: None,
+        enable_git_worktree: None,
     };
 
     let cli_overrides = cli_overrides