An agent asking `forges` sees each child's size and usage, as the machines page shows them #10903

Intent: the owner's agent can tell which client forge is near its size without opening a page. #10875 put each child's usage against its size on /system (src/spawn/usage.ts); the MCP forges tool still answers name, address, whom for, apps, claimed and last activity only.

Done when:

• [x] each item of MCP forges carries size and usage (disk, memory, CPU as the page reads them; null when unknown), from the same function the page uses
• [x] the REST route the tool calls returns the same fields and the OpenAPI description names them (test/openapi drift test stays green)
• [x] a test spawns a child fixture and reads the fields through MCP
• [x] bun test test/mcp-size.test.ts passes without raising a ceiling: keep the doc line short

Files: src/http/mcp/tools/forges.ts, src/spawn/usage.ts (read only), the forges REST route, a new test.

Parked by the crew: the check stayed red through 2 automatic tries (pull #10917 closed; its branch torque/10903-an-agent-asking-forges-sees-each-child-s at c9c902bd808f keeps the work so far). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

try 1: src/contract/openapi/routes/getApiV1Forges.ts, src/http/mcp/tools/forges.ts, src/spawn/clients.ts, src/spawn/usage.ts, test/mcp-forges-usage.test.ts try 2: docs/MCP.md, test/mcp-forges-usage.test.ts try 3: test/mcp-forges-usage.test.ts

Reviewed pull #10957 at its head: ticked each item of MCP forges carries size and usage (disk, memory, CPU as the page reads them; null when unknown), from the same function the page uses; the REST route the tool calls returns the same fields and the OpenAPI description names them (test/openapi drift test stays green); a test spawns a child fixture and reads the fields through MCP; bun test test/mcp-size.test.ts passes without raising a ceiling: keep the doc line short.