MCP `new_app`: make an app from a sentence and get back its URL, its first issue and who is on it #3671

For building apps fast from any harness: POST /api/v1/apps (src/http/api.ts) and the New app page both call newApp in src/actions/apps.ts, but the forge's MCP server (src/http/mcp.ts) has no tool for it. An agent in Claude Code or OpenCode cannot start an app without leaving for the page.

Done:

• [ ] A new_app tool takes the same inputs as POST /api/v1/apps (description, name, owner, visibility, agent or pick) and goes through newApp. It answers with the repository, the app's URL, the first issue's number and the agent on it.
• [ ] The same refusals as the API, in the same words: unverified, app_limit, no_such_agent, a name already taken. It is refused without the rw scope.
• [ ] docs/MCP.md lists the tool. The test that keeps tools/list small still passes.
• [ ] A test makes an app through MCP and reads it back with the apps tool.

Files: src/http/mcp.ts, docs/MCP.md, a test under test/. Fits: whoever has done the recent MCP tools.