Add conventional-commits plugin to marketplace - #134

Add conventional-commits

Adds a single catalog entry for conventional-commits, a small skills-only plugin that helps Grok Build write Conventional Commits 1.0.0 messages and derive changelogs from a staged diff.

• Source: https://github.com/covisualize/conventional-commits (remote, pinned SHA 07d51e97…)
• License: MIT
• No executable code / MCP / hooks / network — it ships one SKILL.md that only guides commit-message writing, so it requires zero runtime permissions.

Checklist

• One entry added to .grok-plugin/marketplace.json, valid JSON, name kebab-case and unique
• Remote source pins a full 40-char lowercase commit SHA; commit is public and reachable
• .grok-plugin/plugin-index.json regenerated (scripts/generate-plugin-index.py) and committed
• homepage, clear description, product-scoped keywords, and a category (development)
• Plugin is licensed (MIT) and the license is stated
• Local validation passes: validate-catalog.py → "Catalog OK", generate-plugin-index.py --check → "Plugin index OK"
• Read and complies with Security expectations (no RCE, no exfiltration; skill-only)