The owner's phone hears at 70% and 80% disk on each machine the forge uses, and /system names the five largest directories #11800

Part of #11686, item 6.

Done:

• [ ] A vitals need at 70% and another at 80% of the disk under the forge's data used, paged to the owner once per crossing and Recovered once (the pager in src/crew/incidents.ts already owns disk-floor).
• [ ] /system (and the MCP system tool) lists the five largest directories under the forge's data with their sizes, read at most once a minute and never on the request path for longer than 0.8 s.
• [ ] Tests: 69% says nothing, 70% pages once, 80% pages once more, falling back below 70% says Recovered; the top-five list orders by size.

Files: src/kernel/vitals.ts, src/crew/incidents.ts, src/web/pages/system.ts, src/http/mcp/tools/system.ts