docs/MCP.md describes the capacity and limits tools: `capacity`, `limits`, `quotas`, `sizes` #5284

Intent: a person or harness reading docs/MCP.md finds every tool the server lists. Today these tools answer in tools/list but have no row in the docs/MCP.md table.

Done:

• [ ] docs/MCP.md's tool table has one row for each of capacity, limits, quotas, sizes, beside its neighbours by theme, saying in plain words what it does and ticking writes when it changes anything, matching the tool's description and handler in src/http/mcp.ts
• [ ] if test/mcp-docs.test.ts exists by then (another issue adds it with an allowlist of undocumented tools, one name per line), these names leave its allowlist; otherwise nothing else changes
• [ ] no source file changes

Files: docs/MCP.md, and the allowlist lines in test/mcp-docs.test.ts if it exists. Other issues add other rows in parallel: add only these.