Migrate docs-cherry-pick from command to skill with backing script

Move cherry-pick logic from 631 lines of prose (command) into a bash
script that handles all deterministic operations: PR info fetching,
branch audit, cherry-pick execution, commit, push, and PR description
generation. The SKILL.md is a thin wrapper (123 lines) covering only
conflict resolution rules that require LLM judgement.

- Delete commands/docs-cherry-pick.md
- Add skills/docs-cherry-pick/SKILL.md
- Add skills/docs-cherry-pick/scripts/cherry_pick.sh

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

and counted toward included files rather than excluded.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kquinn1204