[BUG] /resume inside worktree session switches cwd to main repo

Bug Description

When using /resume inside a worktree session to resume a previous session from the same worktree, the cwd switches to the main git repository root instead of staying in the worktree.

Steps to Reproduce

1. In a git repo, start a worktree session: claude -w my-feature
2. Claude creates a worktree and starts the session in it
3. Do some work, create a commit, then exit the session while keeping the worktree
4. Start a new session in the same worktree: claude -w my-feature
5. Confirm cwd is the worktree directory -- this works correctly
6. Use /resume to resume the previous session from step 2
7. Observe that cwd changes to the main repo root instead of staying in the worktree

Expected Behavior

After /resume, the cwd should remain in (or switch back to) the worktree directory, per the v2.1.81 changelog: "Resuming a session that was in a worktree now switches back to that worktree."

Actual Behavior

The cwd switches to the main git repository root after /resume.

Environment Info

• Platform: macOS (darwin)
• Version: 2.1.96
• Terminal: Ghostty