feat(skill-writer): skill-writer clean subcommand

[pixelastic]

Parent

#28

What to build

Add the clean subcommand to the skill-writer script.

skill-writer clean --scope <id> must:

• Remove the git worktree at ~/local/tmp/claude/skill-writer/<scope>/worktree/ (git worktree remove --force)
• Delete the branch skill-writer/<scope> from the source repo
• Remove the scope directory ~/local/tmp/claude/skill-writer/<scope>/ entirely (rm -rf)

The source repo for the branch deletion must be resolved from the worktree itself (via git -C <worktree> rev-parse --git-common-dir) so clean works without requiring --dir.

Acceptance criteria

• skill-writer clean --scope foo removes ~/local/tmp/claude/skill-writer/foo/ entirely
• Branch skill-writer/foo is deleted from the source repo
• Running clean on a non-existent scope produces a clear error on stderr and exits non-zero
• Running clean when the worktree is already detached or missing does not crash

Blocked by

• feat(skill-writer): skill-writer test core #29