feat: add git wt checkout for existing branch worktrees

[kuderr]

Summary

• New git wt checkout <branch> [name] command (alias co) to check out existing local or remote branches into managed worktrees
• Smart branch resolution: local branches, explicit remote refs (origin/fix/bug), and auto-detection from remotes
• Worktree names derived from branch names (feature/login → feature-login, remote prefix stripped)
• wtco shell alias (checkout + cd), completions for bash & zsh
• --copy-env and --copy-ai flags supported

Test plan

• make lint — shellcheck passes
• git wt checkout <local-branch> — creates worktree with existing local branch
• git wt checkout origin/<branch> — creates tracking branch and worktree (falls back to local if tracking branch already exists)
• git wt checkout <branch-on-remote> — auto-detects from remote
• git wt checkout <branch> <custom-name> — custom worktree name
• git wt checkout nonexistent — clear error message
• git wt rm works on checkout-created worktrees
• git wt help shows checkout command and options

🤖 Generated with Claude Code

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