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/skills/customize/SKILL.md

@@ -9,10 +9,15 @@ This skill helps users add capabilities or modify behavior. Use AskUserQuestion
 
 ## Workflow
 
-1. **Understand the request** - Ask clarifying questions
-2. **Plan the changes** - Identify files to modify
-3. **Implement** - Make changes directly to the code
-4. **Test guidance** - Tell user how to verify
+1. **Install marketplace** - If feature skills aren't available yet, install the marketplace plugin:
+   ```bash
+   claude plugin install nanoclaw-skills@nanoclaw-skills --scope project
+   ```
+   This is hot-loaded — all feature skills become immediately available.
+2. **Understand the request** - Ask clarifying questions
+3. **Plan the changes** - Identify files to modify. If a skill exists for the request (e.g., `/add-telegram` for adding Telegram), invoke it instead of implementing manually.
+4. **Implement** - Make changes directly to the code
+5. **Test guidance** - Tell user how to verify
 
 ## Key Files