feat: add -x/--exec option to run commands in worktree after setup

[avihut]

Summary

• Adds -x/--exec repeatable option to checkout, checkout-branch, clone, and init commands
• Commands run sequentially in the worktree directory after hooks complete, with full stdio inheritance (interactive apps like claude or vim work directly)
• On failure, stops execution and propagates the error, but always writes cd_path so the shell still lands in the worktree

Usage

git worktree-checkout-branch my-feature -x claude
git worktree-clone https://github.com/org/repo -x 'mise install' -x 'npm run dev'
git worktree-init my-project -x 'echo setup complete'

Test plan

• Unit tests pass (349 tests)
• Integration tests pass (8 new exec-specific tests covering single/multiple commands, failure propagation, all four commands, existing worktree case)
• mise run clippy — zero warnings
• mise run fmt — clean
• Man pages regenerated and verified
• CLI docs regenerated and verified
• SKILL.md updated with exec option documentation

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