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/update-skills/SKILL.md

@@ -0,0 +1,130 @@
+---
+name: update-skills
+description: Check for and apply updates to installed skill branches from upstream.
+---
+
+# About
+
+Skills are distributed as git branches (`skill/*`). When you install a skill, you merge its branch into your repo. This skill checks upstream for newer commits on those skill branches and helps you update.
+
+Run `/update-skills` in Claude Code.
+
+## How it works
+
+**Preflight**: checks for clean working tree and upstream remote.
+
+**Detection**: fetches upstream, lists all `upstream/skill/*` branches, determines which ones you've previously merged (via merge-base), and checks if any have new commits.
+
+**Selection**: presents a list of skills with available updates. You pick which to update.
+
+**Update**: merges each selected skill branch, resolves conflicts if any, then validates with build + test.
+
+---
+
+# Goal
+Help users update their installed skill branches from upstream without losing local customizations.
+
+# Operating principles
+- Never proceed with a dirty working tree.
+- Only offer updates for skills the user has already merged (installed).
+- Use git-native operations. Do not manually rewrite files except conflict markers.
+- Keep token usage low: rely on `git` commands, only open files with actual conflicts.
+
+# Step 0: Preflight
+
+Run:
+- `git status --porcelain`
+
+If output is non-empty:
+- Tell the user to commit or stash first, then stop.
+
+Check remotes:
+- `git remote -v`
+
+If `upstream` is missing:
+- Ask the user for the upstream repo URL (default: `https://github.com/qwibitai/nanoclaw.git`).
+- `git remote add upstream <url>`
+
+Fetch:
+- `git fetch upstream --prune`
+
+# Step 1: Detect installed skills with available updates
+
+List all upstream skill branches:
+- `git branch -r --list 'upstream/skill/*'`
+
+For each `upstream/skill/<name>`:
+1. Check if the user has merged this skill branch before:
+   - `git merge-base --is-ancestor upstream/skill/<name>~1 HEAD` — if this succeeds (exit 0) for any ancestor commit of the skill branch, the user has merged it at some point. A simpler check: `git log --oneline --merges --grep="skill/<name>" HEAD` to see if there's a merge commit referencing this branch.
+   - Alternative: `MERGE_BASE=$(git merge-base HEAD upstream/skill/<name>)` — if the merge base is NOT the initial commit and the merge base includes commits unique to the skill branch, it has been merged.
+   - Simplest reliable check: compare `git merge-base HEAD upstream/skill/<name>` with `git merge-base HEAD upstream/main`. If the skill merge-base is strictly ahead of (or different from) the main merge-base, the user has merged this skill.
+2. Check if there are new commits on the skill branch not yet in HEAD:
+   - `git log --oneline HEAD..upstream/skill/<name>`
+   - If this produces output, there are updates available.
+
+Build three lists:
+- **Updates available**: skills that are merged AND have new commits
+- **Up to date**: skills that are merged and have no new commits
+- **Not installed**: skills that have never been merged
+
+# Step 2: Present results
+
+If no skills have updates available:
+- Tell the user all installed skills are up to date. List them.
+- If there are uninstalled skills, mention them briefly (e.g., "3 other skills available in upstream that you haven't installed").
+- Stop here.
+
+If updates are available:
+- Show the list of skills with updates, including the number of new commits for each:
+  ```
+  skill/<name>: 3 new commits
+  skill/<other>: 1 new commit
+  ```
+- Also show skills that are up to date (for context).
+- Use AskUserQuestion with `multiSelect: true` to let the user pick which skills to update.
+  - One option per skill with updates, labeled with the skill name and commit count.
+  - Add an option: "Skip — don't update any skills now"
+- If user selects Skip, stop here.
+
+# Step 3: Apply updates
+
+For each selected skill (process one at a time):
+
+1. Tell the user which skill is being updated.
+2. Run: `git merge upstream/skill/<name> --no-edit`
+3. If the merge is clean, move to the next skill.
+4. If conflicts occur:
+   - Run `git status` to identify conflicted files.
+   - For each conflicted file:
+     - Open the file.
+     - Resolve only conflict markers.
+     - Preserve intentional local customizations.
+     - `git add <file>`
+   - Complete the merge: `git commit --no-edit`
+
+If a merge fails badly (e.g., cannot resolve conflicts):
+- `git merge --abort`
+- Tell the user this skill could not be auto-updated and they should resolve it manually.
+- Continue with the remaining skills.
+
+# Step 4: Validation
+
+After all selected skills are merged:
+- `npm run build`
+- `npm test` (do not fail the flow if tests are not configured)
+
+If build fails:
+- Show the error.
+- Only fix issues clearly caused by the merge (missing imports, type mismatches).
+- Do not refactor unrelated code.
+- If unclear, ask the user.
+
+# Step 5: Summary
+
+Show:
+- Skills updated (list)
+- Skills skipped or failed (if any)
+- New HEAD: `git rev-parse --short HEAD`
+- Any conflicts that were resolved (list files)
+
+If the service is running, remind the user to restart it to pick up changes.