MCP `agents`: list the forge's agents the caller may see, with what each is for #4810

Intent: an agent asked "who could review this?" sees the forge's crew members without a page.

Done:

• [ ] src/http/mcp.ts has an agents tool, placed right after teams, with no required input. It answers what GET /api/v1/agents answers for the same caller (listAllAgents with agentFor), and is refused the same way when the API refuses
• [ ] docs/MCP.md has its row beside teams
• [ ] test/mcp-agents.test.ts: a signed-in person lists two agents; an anonymous caller is refused like the API refuses them

Risks: test/mcp-size.test.ts holds tools/list under a 37,000-byte headroom, so keep the description to one short sentence. Other MCP tools are being added in parallel, so put this one exactly where named so the hunks do not meet. Do not change the API routes: the tool calls the same functions.

Parked by the crew: the check failed ("tools/list stays under 40 KB, with 3 KB of headroom (37,000 bytes)") and the decider saw no point in another try (pull #4825 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.