The MCP size test says how many bytes are left, and docs/MCP.md says a new door extends a tool before it becomes one #5033

Intent: the next builder knows before pushing whether a new MCP tool fits, and reaches for an action or kind on an existing tool first.

Done:

• [ ] test/mcp-size.test.ts's failure message names the bytes over and the bytes the new answer has, and on a pass the test file's header comment explains how to measure locally (bun test test/mcp-size.test.ts)
• [ ] docs/MCP.md says, beside the tool table: tools/list has a 40,000-byte ceiling and a 37,000-byte headroom; a new door is an action/kind on the tool that already owns the resource before it is a new tool; a new tool's description is one sentence; the ceiling is the owner's and is never raised in a feature pull
• [ ] the ceilings do not change

Files: test/mcp-size.test.ts, docs/MCP.md. Risk: none; touch no tool.