Add post-bridge plugin (remote source)

[jackfriks]

What this PR does

Adds the Post Bridge plugin as a remote-source entry.

• Plugin name: post-bridge
• Type: remote source
• Source URL + pinned SHA: https://github.com/post-bridge-hq/agent-mode.git @ 681e8e6dac94eb6d9c0467e303e158dea8ba2958
• Homepage: https://www.post-bridge.com

Post Bridge is a social media scheduler: the plugin bundles skills, the postbridge-cli npm package, and a hosted MCP server so agents can create, schedule and publish posts to 9 platforms (Instagram, TikTok, YouTube, X, LinkedIn, Facebook, Pinterest, Threads, Bluesky), upload media, and read analytics. MIT licensed; manifest is .claude-plugin/plugin.json plus the Agent Plugins plugin.json/mcp.json at the repo root.

Ownership

• I own this plugin or have the right to distribute it.
• The source repo is published under our official org (post-bridge-hq).

Checklist

• Added/updated exactly one entry in .grok-plugin/marketplace.json (valid JSON, kebab-case name).
• Remote source pins a full 40-char lowercase commit sha, and that commit is public + reachable.
• Regenerated .grok-plugin/plugin-index.json (python3 scripts/generate-plugin-index.py).
• python3 scripts/validate-catalog.py passes locally.
• python3 scripts/generate-plugin-index.py --check passes locally.

🤖 Generated with Claude Code