feat: add -w, --worktree [name] flag for isolated sessions

[rrva]

Problem

Users running multiple Kimi Code sessions on the same repository need a way to avoid file conflicts and branch-switching overhead. The legacy Kimi CLI already supports this with a worktree flag, and users expect the same capability in Kimi Code CLI.

Proposed solution

Add a -w, --worktree [name] CLI flag that creates a new git worktree for the session:

• New worktrees live under <repo-root>/.kimi/worktrees/<name> with a detached HEAD checkout at the current HEAD.
• Auto-generate a three-word slug name when no name is supplied.
• Reject combinations with --session or --continue.
• Persist worktree_path and parent_repo_path in session metadata.
• Clean up the worktree on exit when the session is empty.

Related PR

• feat(kimi-code): add -w, --worktree [name] flag for isolated sessions #818