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_config.rs

@@ -165,6 +165,7 @@ impl CodexToolCallParam {
             include_view_image_tool: None,
             show_raw_agent_reasoning: None,
             tools_web_search_request: None,
+            enable_git_worktree: None,
         };
 
         let cli_overrides = cli_overrides