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>

scripts/run-migrations.ts

@@ -3,7 +3,15 @@ import { execFileSync, execSync } from 'child_process';
 import fs from 'fs';
 import path from 'path';
 
-import { compareSemver } from '../skills-engine/state.js';
+function compareSemver(a: string, b: string): number {
+  const partsA = a.split('.').map(Number);
+  const partsB = b.split('.').map(Number);
+  for (let i = 0; i < Math.max(partsA.length, partsB.length); i++) {
+    const diff = (partsA[i] || 0) - (partsB[i] || 0);
+    if (diff !== 0) return diff;
+  }
+  return 0;
+}
 
 // Resolve tsx binary once to avoid npx race conditions across migrations
 function resolveTsx(): string {