feat(agent): emit templated alerts when AI is unavailable

[hoalongnatsu]

Detection is already AI-free (the brain's SignalDetector), but emitDetect
dropped the alert whenever AI was off, quota-limited, or errored — so an
operator with no AI key got silent detection. Build a deterministic
templated finding on those paths and send it through the existing emitter,
so a brain-detected anomaly is never silently dropped. AI becomes an
enrichment layer, not the alerting gate.

• new deterministic_finding.go: pure builder (title/summary/severity/
  confidence) from the AgentResult + deviation sigma, redacted samples only
• emitDetect: AI-off/quota/error now emit (emitted_basic / _quota / _error);
  AI-success + cache paths unchanged
• audit: Model="heuristic" + heuristic marker distinguish templated incidents
• remove the misleading "no incidents will be emitted" boot WARN
• docs: detect mode pages with or without an AI key