fix(worktree): fetch base branch before creating worktree

[aletc1]

Summary

• Worktrees were being created off origin/<baseBranch>, a local remote-tracking ref that only reflects the last git fetch. When the remote had advanced, the new worktree was born stale and caused avoidable merge conflicts later when users tried to merge or open a PR.
• createWorktreeForChat now best-effort-fetches origin <baseBranch> before calling git worktree add, using the existing createGitForNetwork (2-min timeout) wrapped in withGitLock for consistency with other fetch call sites.
• Fetch failures are caught and logged via console.warn — worktree creation proceeds unchanged, so offline/auth/missing-remote scenarios behave exactly as before.
• Skipped when branchType === "local" (user opted into a local ref) and when the repo has no origin remote.

Test plan

• Happy path — remote ahead of local: push a commit to origin/main from another clone, then create a chat with useWorktree: true, base main. Verify the worktree's HEAD matches the just-pushed remote commit (git -C ~/.21st/worktrees/<slug>/<folder> rev-parse HEAD).
• Offline / fetch failure: disable the network (or point origin at a bad URL) and create a chat. Expect [worktree] Pre-create fetch of origin/<baseBranch> failed: … in the main-process console, worktree still created against cached origin/<baseBranch>, no user-facing error.
• No origin remote: in a repo without origin, create a chat. Expect no fetch attempt, no warning, worktree created off local branch as before.
• branchType === "local": pick an explicit local branch when creating a chat. Expect no fetch attempt; worktree branches from the chosen local ref unchanged.
• Concurrency: create two chats back-to-back against the same project — lock via withGitLock should serialize the two fetches cleanly.