Research and add Antigravity connector support

Parent: #76

## Goal

Determine the correct Antigravity integration surface and add generated connector output only after the official path is known.

## Research questions

- Does Antigravity consume VS Code Marketplace extensions directly?
- Does it support Open VSX or a separate extension registry?
- Does it have a documented MCP config path?
- Does it support rules, skills, plugins, or workspace instructions?
- Can the shared `wordpress-studio` and `wordpress-telemetry` MCP setup be represented in its native format?

## Implementation path

- Document official references before adding generated files.
- Add Antigravity generation to `scripts/build-plugins.mjs` only for supported native artifacts.
- Add verifier coverage in `scripts/verify-plugins.mjs`.
- Add docs and manual test steps for installation/configuration.

## Acceptance criteria

- Official integration surface is documented with source links.
- Unsupported assumptions are explicitly ruled out.
- Generated output exists only if Antigravity has a documented native path.
- `pnpm build` and `pnpm verify` pass after implementation.

## AI assistance

- **AI assistance:** Yes
- **Tool(s):** OpenCode (GPT-5.5)
- **Used for:** Drafting issue scope and acceptance criteria from planning discussion.

Start by researching Antigravity's official extension, registry, MCP, rules, skills, plugins, and workspace-instruction documentation, recording source links and unsupported assumptions. Then inspect scripts/build-plugins.mjs and scripts/verify-plugins.mjs to determine whether native generated artifacts are possible. Done means documented integration references, verifier coverage, installation steps, and passing pnpm build and pnpm verify.

Written by the indexing model from the issue text.