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/add-reactions/SKILL.md

@@ -1,103 +0,0 @@
----
-name: add-reactions
-description: Add WhatsApp emoji reaction support — receive, send, store, and search reactions.
----
-
-# Add Reactions
-
-This skill adds emoji reaction support to NanoClaw's WhatsApp channel: receive and store reactions, send reactions from the container agent via MCP tool, and query reaction history from SQLite.
-
-## Phase 1: Pre-flight
-
-### Check if already applied
-
-Read `.nanoclaw/state.yaml`. If `reactions` is in `applied_skills`, skip to Phase 3 (Verify). The code changes are already in place.
-
-## Phase 2: Apply Code Changes
-
-Run the skills engine to apply this skill's code package. The package files are in this directory alongside this SKILL.md.
-
-### Apply the skill
-
-```bash
-npx tsx scripts/apply-skill.ts .claude/skills/add-reactions
-```
-
-This deterministically:
-- Adds `scripts/migrate-reactions.ts` (database migration for `reactions` table with composite PK and indexes)
-- Adds `src/status-tracker.ts` (forward-only emoji state machine for message lifecycle signaling, with persistence and retry)
-- Adds `src/status-tracker.test.ts` (unit tests for StatusTracker)
-- Adds `container/skills/reactions/SKILL.md` (agent-facing documentation for the `react_to_message` MCP tool)
-- Modifies `src/db.ts` — adds `Reaction` interface, `reactions` table schema, `storeReaction`, `getReactionsForMessage`, `getMessagesByReaction`, `getReactionsByUser`, `getReactionStats`, `getLatestMessage`, `getMessageFromMe`
-- Modifies `src/channels/whatsapp.ts` — adds `messages.reaction` event handler, `sendReaction()`, `reactToLatestMessage()` methods
-- Modifies `src/types.ts` — adds optional `sendReaction` and `reactToLatestMessage` to `Channel` interface
-- Modifies `src/ipc.ts` — adds `type: 'reaction'` IPC handler with group-scoped authorization
-- Modifies `src/index.ts` — wires `sendReaction` dependency into IPC watcher
-- Modifies `src/group-queue.ts` — `GroupQueue` class for per-group container concurrency with retry
-- Modifies `container/agent-runner/src/ipc-mcp-stdio.ts` — adds `react_to_message` MCP tool exposed to container agents
-- Records the application in `.nanoclaw/state.yaml`
-
-### Run database migration
-
-```bash
-npx tsx scripts/migrate-reactions.ts
-```
-
-### Validate code changes
-
-```bash
-npm test
-npm run build
-```
-
-All tests must pass and build must be clean before proceeding.
-
-## Phase 3: Verify
-
-### Build and restart
-
-```bash
-npm run build
-```
-
-Linux:
-```bash
-systemctl --user restart nanoclaw
-```
-
-macOS:
-```bash
-launchctl kickstart -k gui/$(id -u)/com.nanoclaw
-```
-
-### Test receiving reactions
-
-1. Send a message from your phone
-2. React to it with an emoji on WhatsApp
-3. Check the database:
-
-```bash
-sqlite3 store/messages.db "SELECT * FROM reactions ORDER BY timestamp DESC LIMIT 5;"
-```
-
-### Test sending reactions
-
-Ask the agent to react to a message via the `react_to_message` MCP tool. Check your phone — the reaction should appear on the message.
-
-## Troubleshooting
-
-### Reactions not appearing in database
-
-- Check NanoClaw logs for `Failed to process reaction` errors
-- Verify the chat is registered
-- Confirm the service is running
-
-### Migration fails
-
-- Ensure `store/messages.db` exists and is accessible
-- If "table reactions already exists", the migration already ran — skip it
-
-### Agent can't send reactions
-
-- Check IPC logs for `Unauthorized IPC reaction attempt blocked` — the agent can only react in its own group's chat
-- Verify WhatsApp is connected: check logs for connection status