feat: add Gemini CLI + OpenClaw support, fix Codex missing 25 skills

Gemini CLI:
- Add GEMINI.md with activation instructions
- Add scripts/gemini-install.sh setup script
- Add scripts/sync-gemini-skills.py (194 skills indexed)
- Add .gemini/skills/ with symlinks for all skills, agents, commands
- Remove phantom medium-content-pro entries from sync script
- Add top-level folder filter to prevent gitignored dirs from leaking

Codex CLI:
- Fix sync-codex-skills.py missing "engineering" domain (25 POWERFUL skills)
- Regenerate .codex/skills-index.json: 124 → 149 skills
- Add 25 new symlinks in .codex/skills/

OpenClaw:
- Add OpenClaw installation section to INSTALLATION.md
- Add ClawHub install + manual install + YAML frontmatter docs

Documentation:
- Update INSTALLATION.md with all 4 platforms + accurate counts
- Update README.md: "three platforms" → "four platforms" + Gemini quick start
- Update CLAUDE.md with Gemini CLI support in v2.1.1 highlights
- Update SKILL-AUTHORING-STANDARD.md + SKILL_PIPELINE.md with Gemini steps
- Add OpenClaw + Gemini to installation locations reference table

Marketplace: all 18 plugins validated — sources exist, SKILL.md present

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/sync-codex-skills.py

@@ -28,6 +28,10 @@ SKILL_DOMAINS = {
         "category": "engineering",
         "description": "Software engineering and technical skills"
     },
+    "engineering": {
+        "category": "engineering-advanced",
+        "description": "Advanced engineering skills - agents, RAG, MCP, CI/CD, databases, observability"
+    },
     "product-team": {
         "category": "product",
         "description": "Product management and design skills"