The fetch before `git worktree add` inherited the 30s default command
timeout. On repositories with thousands of commits a day, fetching the base
branch after a few days away legitimately takes longer, so worktree creation
failed with "Git command timed out." every time.

Give fetchRemote the same 5-minute bound `git worktree add` got in #6326.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>