Report Core process metrics and node host history

[SaladDay]

Core metrics previously exposed only Go heap memory, and node dialogs had no host CPU or retained host history. This adds process CPU/RSS and cgroup limits to the existing Core metrics response, plus an administrator-only node detail read with real heartbeat observations and 1h/6h/24h history.

Both reuse existing sampling and storage: Core's 30-second ring and the Runtime sweep with PostgreSQL seven-day retention. Missing measurements, initial CPU samples and offline buckets remain null. The node list and public Agents API schemas, enrollment/capacity policy and frontend pages are unchanged. API contracts, generated admin OpenAPI and SQLC, and architecture documentation are updated.

Validation:

• Final 7192e3f: full Linux make check passed on mx1, including database tests, 610 client tests, 332 Web unit tests, 12 browser tests and executor checks. The private test copy used Chromium; the existing optional MiniMax packaged-tools test remained skipped.
• Focused race, API authentication, heartbeat fencing, history aggregation/nulls and retention tests passed; generated public schema is unchanged.
• Real isolated Linux Core/node/HTTP/PostgreSQL acceptance passed: process counters, actual 1.5-core/512-MiB cgroup limits, node host observations, complete buckets, disconnect gaps, reconnect and restart persistence. Final code also passed an actual root-cgroup probe. Owned services were stopped after acceptance.
• Fresh GPT-6 Astra high full-diff blind review completed; its one root-cgroup fallback finding was corrected and passed focused reviewer verification and real Linux regression.

The first full check failed on macOS metadata accidentally copied into its private test tree; rerunning the entire gate from a clean Git archive passed. Seven-day aging uses database fixtures; no seven-day wall-clock observation or model execution is claimed. Latest main #101/#106 was checked for overlap; neither changes this batch's backend files. Existing demonstration services were not upgraded.