feat: skills as branches, channels as forks

Replace the custom skills engine with standard git operations.
Feature skills are now git branches (on upstream or channel forks)
applied via `git merge`. Channels are separate fork repos.

- Remove skills-engine/ (6,300+ lines), apply/uninstall/rebase scripts
- Remove old skill format (add/, modify/, manifest.yaml) from all skills
- Remove old CI (skill-drift.yml, skill-pr.yml)
- Add merge-forward CI for upstream skill branches
- Add fork notification (repository_dispatch to channel forks)
- Add marketplace config (.claude/settings.json)
- Add /update-skills operational skill
- Update /setup and /customize for marketplace plugin install
- Add docs/skills-as-branches.md architecture doc

Channel forks created: nanoclaw-whatsapp (with 5 skill branches),
nanoclaw-telegram, nanoclaw-discord, nanoclaw-slack, nanoclaw-gmail.

Upstream retains: skill/ollama-tool, skill/apple-container, skill/compact.

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

CLAUDE.md

@@ -57,7 +57,7 @@ systemctl --user restart nanoclaw
 
 ## Troubleshooting
 
-**WhatsApp not connecting after upgrade:** WhatsApp is now a separate skill, not bundled in core. Run `/add-whatsapp` (or `npx tsx scripts/apply-skill.ts .claude/skills/add-whatsapp && npm run build`) to install it. Existing auth credentials and groups are preserved.
+**WhatsApp not connecting after upgrade:** WhatsApp is now a separate channel fork, not bundled in core. Run `/add-whatsapp` (or `git remote add whatsapp https://github.com/qwibitai/nanoclaw-whatsapp.git && git fetch whatsapp main && git merge whatsapp/main && npm run build`) to install it. Existing auth credentials and groups are preserved.
 
 ## Container Build Cache