MCP `agent`: create, rename and remove the caller's own agents, with the same rules as the API #4901

Intent: a person makes a new crew member from their harness.

Done:

• [ ] src/http/mcp.ts has a agent tool, placed right after spawn, that calls what POST /api/v1/agents, PATCH and DELETE /api/v1/agents/:username call, for the same caller
• [ ] docs/MCP.md has its row beside spawn
• [ ] test/mcp-agent.test.ts: a person creates an agent, renames it and removes it; another person cannot remove it; a taken name is refused with the API's error

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. Do not change the API routes: the tool calls the functions they call, with the same permissions.