MCP `economics`: the slider's position, the policy it means, and cost, revenue and gap

Agents (and the governor's twin) read the economics the way they read usage.

Acceptance:

[ ] MCP tool economics (writes: false): position, policyAt(position) in plain fields, this month's and today's cost / revenue / gap forge-wide, and per app when repo is given (refused unless the caller can write it)
[ ] the same JSON at GET /api/v1/forge/economics/summary and GET /api/v1/repos/:owner/:name/economics
[ ] documented in docs/MCP.md
[ ] tests: the MCP answer equals the API answer; a reader without write on the app is refused

Files: src/http/mcp.ts, src/http/api.ts, docs/MCP.md.

Part of the profit slider (#392); the shape is docs/PROFIT.md.

MCP `economics`: the slider's position, the policy it means, and cost, revenue and gap — read-only #511