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-LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2025–2026 tommy0103
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.