Initial t3bootstrap-skills plugin: three skills for T3Bootstrap on TYPO3 v14

- t3bootstrap-site-package: composer + site sets + when to add a local extension
- t3bootstrap-overrides: 4-layer override surface (Settings → SCSS → Fluid → DataProcessor)
- t3bootstrap-content-elements: 23 t3bs_* CE catalog + migration matrix (HTML/WP/Joomla/older TYPO3) + Flux-bs5 upgrade wizard

Mirrors the structure of wapplersystems/typo3-skills (.claude-plugin/{plugin.json,marketplace.json}, .agents/skills/<name>/{SKILL.md,agents/openai.yaml}).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,14 @@
+# macOS metadata files
+.DS_Store
+._*
+.Spotlight-V100
+.Trashes
+.fseventsd/
+Icon?
+
+# IDE
+.idea/
+.vscode/
+
+# OS
+Thumbs.db