MCP tools/list has 3 KB of headroom: the longest tool descriptions say the same in fewer words #4162

Part one of #3340. Today an admin's tools/list is 39,977 bytes against the 40,000 ceiling in test/mcp-size.test.ts and test/mcp-pulls.test.ts, so any new tool turns the check red (that is what parked #3340's first pull).

Done:

• [ ] An admin's tools/list is at most 37,000 bytes. The ceiling in both tests stays 40,000.
• [ ] Only descriptions change: the longest (spawn, plan, limb, cloud_account, new_app, tell, feed, limits) are cut to what an agent needs to pick the tool and call it right. No tool is removed or renamed, and no input schema changes.
• [ ] A test asserts the admin's tools/list is at most 37,000 bytes, so the headroom holds.

Files: src/http/mcp.ts, test/mcp-size.test.ts. Before pushing: bun x tsc --noEmit, bun test test/mcp-size.test.ts test/mcp-pulls.test.ts test/mcp.test.ts.