Add antigravity MCP install target

[robertjamesprior]

Summary

Adds kernel mcp install --target antigravity, so Antigravity users don't have to hand-edit JSON to reach the Kernel MCP server.

Antigravity keys remote servers off serverUrl and ignores url and httpUrl, so none of the existing HTTP target shapes work for it — this needs its own installer rather than reuse of the Cursor or VS Code path. Config goes to ~/.gemini/config/mcp_config.json, home-relative on every platform, so no per-OS branch is needed.

Written output:

{
  "mcpServers": {
    "kernel": {
      "serverUrl": "https://mcp.onkernel.com/mcp"
    }
  }
}

Uses writeJSONFile (0644/0755) like every target except fx — the file this writes holds no credential. Antigravity stores OAuth tokens separately in ~/.gemini/antigravity/mcp_oauth_tokens.json.

On writing the OAuth shape rather than an API key

This writes the plain OAuth config, matching all eight existing targets. Antigravity's OAuth is currently unreliable — it can finish the browser flow and still send initialize without the bearer token, returning 401 Unauthorized (antigravity-cli#25, open since May).

The alternative was writing an Authorization: Bearer header with a Kernel API key, which works today. I didn't, for two reasons: it diverges from every other target, and it needs the user to already hold a key. This is the same tradeoff --target fx already makes — it writes OAuth config knowing fx's OAuth is broken, and the docs carry the workaround. Worth overriding if you'd rather the CLI optimize for what works today over what's consistent.

Validation

• go build ./..., go vet ./cmd/mcp/, gofmt -l: all clean.
• go test ./...: all packages pass, including two new tests — one covering an existing config (other servers and unrelated keys preserved, url explicitly absent) and one covering a clean machine, which is what exercises creating the two-level .gemini/config directory.
• Ran the built binary against a throwaway HOME: writes the file above, prints the Customizations/paste-code next steps, and appears in --help and the invalid-target error.
• Not driven inside Antigravity itself. The serverUrl key, config path, and token-store location come from Antigravity's MCP documentation.

Merge order

kernel/docs#612 documents this target and currently says no CLI target exists. Land this first, then that.

🤖 Generated with Claude Code

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Note

Low Risk
CLI-only change that writes non-secret local MCP JSON using the same patterns as other install targets; no server or auth logic changes.

Overview
Adds kernel mcp install --target antigravity so Antigravity users get Kernel MCP wired up without hand-editing JSON.

The installer writes ~/.gemini/config/mcp_config.json, merging a kernel entry under mcpServers that uses Antigravity’s serverUrl field (not url / HTTP shapes used by other targets). Help text and post-install steps cover reload, Customizations Authenticate, and pasting the browser authorization code.

Two tests cover merging into an existing config (preserving other servers/settings and ensuring url is not written) and a clean install that creates the config path.

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