ci: package skill as installable zip via Gitea Actions

Add a reproducible way to build the Claude-skill package and publish it:

- scripts/build-skill-zip.sh: stages SKILL.md + README + references/ + assets/
  into a zip whose single top-level cycling-training/ folder is the layout
  Claude Desktop / claude.ai import expects. Validates SKILL.md frontmatter.
- .gitea/workflows/release-skill.yml: on tag v* builds the zip and attaches it
  to a Gitea release (via the API using the automatic token); on manual
  dispatch uploads it as a run artifact.
- README: document the packaging/release flow and Claude Desktop zip-import.
- .gitignore: ignore dist/ build output.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TqrhBhC3GEcKyaw8RTk8G6

.gitignore

@@ -1,6 +1,9 @@
 # Local Claude Code settings (machine-specific)
 .claude/settings.local.json
 
+# Build output (skill zip packages)
+dist/
+
 # OS / editor cruft
 .DS_Store
 *.swp