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/publish-skill.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SKILL_DIR="dist/obelisk-skill"
+REMOTE="git@github.com:tommy0103/obelisk-skill.git"
+
+if [ ! -d "$SKILL_DIR/scripts" ]; then
+  echo "Error: run 'npm run build:skill' first" >&2
+  exit 1
+fi
+
+cp packaging/skill-README.md "$SKILL_DIR/README.md"
+cp packaging/skill-LICENSE "$SKILL_DIR/LICENSE"
+
+cd "$SKILL_DIR"
+rm -rf .git
+git init
+git remote add origin "$REMOTE"
+git add -A
+git commit -m "publish: $(date -u +%Y-%m-%dT%H:%M:%SZ)"
+git push --force origin HEAD:main