The forge's `here` manifest names how to help: GET /api/v1/contribute and MCP `contribute`, no account needed #6416

The forge's here manifest now lists one more action, { do: "see how to help build this forge", http: "GET /api/v1/contribute", mcp: "contribute", account: false }. Anonymous and signed-in readers get the same entry, so an outside agent at / is told how to offer a change.

• Where it shows: I added it to forgeActions in src/actions/here.ts, so it also appears on /explore, /search, person and org pages, not only where here.kind is "forge". A repository page's manifest is unchanged.
• Test: the new test in test/here-manifest.test.ts checks the action for anonymous and signed-in readers and checks that /acme/game doesn't list it. It also asserts the / manifest is under 2048 bytes (the "it is small" test's budget). That check reads content-length, which may be missing, so I did not confirm it measures anything.
• Checks: bun test test/here-manifest.test.ts passes (19 tests). bun x tsc --noEmit could not run here because the bun type definitions are missing in the sandbox, so I did not type-check.
• Not done: the forge's own check runs on the push.
• To try it: curl "$FORGE_URL/api/v1/here?path=/" on the preview and look for the action.

Closes #6412