MCP `telemetry_history`: the machine's CPU, memory and disk over the last hours, beside `system`'s now #4804

Intent: an owner asking "was the box struggling this afternoon?" gets an answer from their harness, not only the value right now.

Done:

• [ ] src/http/mcp.ts has a telemetry_history tool, placed right after system, taking hours (1–72, default 6). It answers what GET /api/v1/telemetry/history answers for the same caller (forge admin or telemetry:read, the same as system)
• [ ] the answer is bounded: at most 300 points a series, downsampled as the API does or in the tool, never the raw table
• [ ] docs/MCP.md has its row beside system
• [ ] test/mcp-telemetry-history.test.ts: an admin reads history with seeded samples; hours outside 1–72 is refused; a person without the scope is refused

Risks: test/mcp-size.test.ts holds tools/list under a 37,000-byte headroom, so keep the description to one short sentence. Other MCP tools are being added in parallel, so put this one exactly where named so the hunks do not meet. Do not change the API routes: the tool calls the same functions.