Add ThumbGate plugin

[IgorGanapolsky]

Summary

• Add ThumbGate as a Cursor plugin for deterministic pre-action checks, thumbs feedback capture, and reliability memory.
• Includes commands, agents, skills, rules, hooks, MCP server config, README, changelog, license, and logo.
• Adds ThumbGate to the root marketplace manifest and README table.

Validation

• node scripts/validate-plugins.mjs
• Result: All plugins validated successfully.

Install after merge

/add-plugin thumbgate

ThumbGate runtime is provided by npx --yes --package thumbgate@latest thumbgate serve through the plugin MCP config.