Adopt Agent Plugins 1.0.0 portable format

[AvdLee]

Summary

• Adds a root plugin.json manifest per the Agent Plugins 1.0.0 specification, the vendor-neutral packaging standard backed by Amazon, Cursor, Microsoft, OpenAI, and Vercel. This repo's skills/ layout already conformed, so unlike the sibling skill repos no folder moves or compatibility symlinks were needed — compatible clients now discover all six skills automatically.
• Fixes version drift: the Claude manifest was at 1.0.1 while the Cursor and Codex manifests were at 1.0.0 — everything now agrees on 1.1.0, and the release workflow bumps every manifest going forward.
• Adds per-skill ChatGPT desktop interface metadata (display names and short descriptions for all six skills) and a homepage field to the Cursor manifest. Updates the README's Cursor section, which still said "coming soon".

Test plan

• Structure validated against the Agent Plugins 1.0.0 spec (closed manifest schema, name constraints, skills/ discovery for all six skills) — 0 errors, 0 warnings
• All wrapper manifests point at existing SKILL.md paths and agree on version 1.1.0
• Load locally in Cursor from ~/.cursor/plugins/local and confirm all six skills are discovered
• Reinstall via Claude Code marketplace and confirm the skills resolve