feat: distribute mymir plugins for native no-clone install

[FrkAk]

Summary

Task Reference: [MYMR-227]

Adds native, no-clone install paths for each supported coding agent. Each brand gets a root marketplace entry that points back into the existing plugins/<cli>/ dirs, so assets are not duplicated and there are no mirror repos.

• Claude Code: root .claude-plugin/marketplace.json with a git-subdir source → plugins/claude-code. claude plugin marketplace add FrkAk/mymir then claude plugin install mymir@mymir.
• Codex: root .agents/plugins/marketplace.json (git-subdir → plugins/codex) for codex plugin marketplace add FrkAk/mymir; the nested plugins/.agents/plugins/marketplace.json stays as the mymir-local contributor marketplace (mirrors Claude's root/local split).
• Cursor: root .cursor-plugin/marketplace.json with source: "plugins/cursor"; Marketplace search and an MCP install deeplink documented.
• Antigravity (new): plugins/antigravity/ bundle: plugin.json, mcp_config.json (uses serverUrl), and skills generated from the Claude canonical by scripts/check-plugins.ts. Global MCP config documented at ~/.gemini/config/mcp_config.json (shared by IDE and CLI); skills installed by copying the bundle into ~/.gemini/config/plugins/.
• Gemini removed: plugins/gemini/ deleted and de-registered from the sync engine; replaced by Antigravity. agy plugin import gemini migration noted.
• Every manifest keeps both named servers: mymir (default https://app.mymir.dev/api/mcp) and mymir-local (http://localhost:3000/api/mcp).
• Plugin version is managed by .version-bump.json + scripts/bump-version.ts (bun run check:version, CI-enforced), covering the four plugin manifests and the MCP server version in lib/mcp/create-server.ts.
• README install section restructured. New tests/plugins/manifests.test.ts validates every manifest and the two-server pattern.

Type of change

• New feature
• Documentation
• Bug fix
• Refactor / cleanup

Testing

• Tested locally with bun run dev
• Linting passes (bun run lint)
• Typecheck passes (bun run typecheck)

bun run test (628 pass / 0 fail), bun run check:plugins (in sync), bun run check:version (aligned), and bun run format:check also pass.

Notes for reviewer

• Install commands reference the published FrkAk/mymir, so remote no-clone install is verifiable only after merge. Locally verified: JSON parses, manifest tests, check:plugins, check:version, full suite, lint, typecheck.
• Codex remote resolution (root marketplace detection + git-subdir source) is the post-merge check; --sparse plugins is documented as a fallback.
• components/home/GetStartedModal.tsx: Gemini entry replaced with Antigravity; the renamed #how-to-set-it-up README anchor updated to #use-the-hosted-version-no-clone.
• Not changed (pre-existing): the Codex ask_user_question substitution in check-plugins.ts (current docs indicate request_user_input).