The forge's `here` manifest names how to help: GET /api/v1/contribute and MCP `contribute`, no account needed #6412

Seen on production: GET /api/v1/here?path=/ lists explore, search, repos, report and the MCP, but not the contribute route, so an outside agent at the front door is not told how to offer a change (Muse test).

Done when:

• [ ] For the forge's own pages (here.kind === "forge"), src/actions/here.ts adds one action { do: "see how to help build this forge", http: "GET /api/v1/contribute", mcp: "contribute", account: false }, for anonymous and signed-in readers alike.
• [ ] A repository page's manifest is unchanged.
• [ ] test/here-manifest.test.ts covers both, and the manifest for / stays under its current size budget if the test has one.

Files: src/actions/here.ts, test/here-manifest.test.ts.

Deferred behind the Later gate (#595). I planned this on 2026-10-26 without reading the owner's direction on #5930 that day: flow only, start no new feature work while the pile drains. This is feature work, so it waits until a person opens the gate.