fix: improve error message when checking out a branch used by another worktree

[draftcode]

Summary

• When CheckoutBranch() fails because the target branch is already checked out in another worktree, detect the git error and return a user-friendly message suggesting git worktree list and cd <worktree-path>
• This improves the error for all commands that use CheckoutBranch(): av switch, av next, av prev, av stack foreach, etc.

Test plan

• Existing unit tests pass (go test ./internal/git/...)
• In a repo with a worktree, try av switch <branch-checked-out-in-worktree> and verify the improved error message appears
• Verify normal checkout (non-worktree) errors are unaffected

🤖 Generated with Claude Code

[aviator-app]

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[aviator-app]

✅ FlexReview Status

Common Owner: aviator-co/engineering (expert-load-balance assignment)
Owner and Assignment:

• ✅ aviator-co/engineering (expert-load-balance assignment)
  • Slack: #av-slack-testing
  • Assigned: simsinght, tulioz
  Owned Files

  • 🔒 internal/git/git.go

Review SLO: 7 business hours if PR size is <= 200 LOC for the first response.

[gemini-code-assist]

Code Review

This pull request introduces automated worktree management for the restack sequencer, enabling it to detach and restore branches in external worktrees and skip branches with dirty working trees. The review feedback suggests improving error diagnostics in IsWorktreeClean by capturing stderr and refactoring the sequencer to store worktree preparation messages in its state so they can be properly displayed in the UI rather than being discarded.

[draftcode]

/aviator merge

[aviator-app]

Aviator has accepted the merge request. It will enter the queue when all of the required status checks have passed. Aviator will update the sticky status comment as the pull request moves through the queue.