MCP `usage` with no repo answers the forge's spend from the forge-wide usage function #8307

Second part of #7656. Today usage requires repo and answers not_found without it.

Done when:

• [ ] repo is optional on usage. Without it, the tool answers the forge-wide function's today, last24h and per-subscription headroom
• [ ] With repo, the answer is unchanged
• [ ] The tool's doc says both forms, and bun test test/mcp-size.test.ts passes without raising a ceiling
• [ ] A test calls usage with no arguments and proves the fields, and that a reader with no writable repository gets an empty answer, not an error

Files: src/http/mcp/tools/usage.ts, test/mcp-usage-forge.test.ts Risk: the tools/list size budget. Keep the doc short.

Done on main already, waiting for you to close it: 64c8cb7f1e365a8150969b786501ae92aa5e02e6

Closed: already done on main in 64c8cb7. Closed by the crew's live triage; reopen it to undo, and it stays open.