ci: auto-publish skill artifact to tommy0103/obelisk-skill on push to main (Phase 7)

Add a GitHub Actions workflow that builds the readable, non-bundled skill
artifact (`npm run build:skill`) and force-pushes it to the dedicated
tommy0103/obelisk-skill public repo whenever the main branch is updated.
The skill repo is a pure derivative — no manual commits, no PRs; the source
of truth stays in tommy0103/obelisk.

- .github/workflows/publish-skill.yml: checkout → npm ci → build:skill →
  clone skill repo → replace content → commit + force push. Auth via
  SKILL_REPO_DEPLOY_KEY (SSH deploy key with write access to obelisk-skill).
- packaging/skill-README.md: the README placed in the skill repo (install
  instructions + link back to source + "auto-published, don't PR here").
- packaging/skill-LICENSE: MIT license for the skill artifact (relicensed
  from the AGPL-3.0 source by the copyright holder).
- packaging/publish-skill.sh: local convenience script for manual publish.
- packaging/skill-package.json: license field updated to MIT.
- README.md: install command updated to tommy0103/obelisk-skill.
- package.json: add publish:skill script.

The skill repo is MIT-licensed for zero adoption friction (local tool, no
library API, no derivative works expected); the source repo stays AGPL-3.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/skill-README.md

@@ -0,0 +1,29 @@
+# Obelisk Skill
+
+Explicit memory infrastructure for coding agents — a queryable SQLite evidence
+layer over local Claude Code and Codex session history.
+
+## Install
+
+```bash
+npx skills add tommy0103/obelisk-skill
+```
+
+Then in any Claude Code session:
+
+```
+/obelisk <your question>
+```
+
+## Source
+
+This repository is **auto-published** from the compiled skill artifact of
+[tommy0103/obelisk](https://github.com/tommy0103/obelisk). Do not open pull
+requests here — contribute to the source repo instead.
+
+## License
+
+MIT — see [LICENSE](LICENSE) in this repository. The
+[source repository](https://github.com/tommy0103/obelisk) is AGPL-3.0; this
+compiled skill artifact is explicitly relicensed under MIT by the copyright
+holder.