Add git claude-wt command for feature worktrees

[viqueen]

Summary

• Add git claude-wt <feature> command that creates a git worktree with a feature/<name> branch
• Feature names with spaces are slugified (e.g. "my cool thing" → feature/my-cool-thing)
• Worktree is created as a sibling directory to the repo (e.g. ../repo-name-feature-slug)

Test plan

• Run git claude-wt "test feature" and verify worktree and branch are created
• Verify spaces in feature names are converted to hyphens

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