skill: add /close-feature + document cross-machine skill setup

- New /close-feature skill graduates .scratch/<slug>/ to docs/ when all
  issues are done; also offers to run proactively when it detects a complete
  feature. Documents machine setup steps (Linux/Mac, Windows, Codex, new skill
  install) in the skill itself.
- Fix AGENTS.md issue-tracker line to reflect the .scratch → docs/ workflow.

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

AGENTS.md

@@ -2,7 +2,7 @@
 
 ### Issue tracker
 
-Issues live as local markdown files under `.scratch/<feature-slug>/`. See `docs/agents/issue-tracker.md`.
+Active feature work lives under `.scratch/<feature-slug>/`. Completed features are merged into `docs/issues/` via `/close-feature`. See `docs/agents/issue-tracker.md`.
 
 ### Triage labels