AI observability: $ai_generation capture over the ORB's review-pipeline AI calls (spike-first)

[JSONbored]

Part of epic #8286 (Phase 3 — Depth). Spike-first, mirroring JSONbored/metagraphed#7763 — but loopover's AI surface is much larger than metagraphed's, so the payoff is too: per-provider/per-model cost, latency, failure, and rate-limit visibility over the review pipeline's real AI spend.

• Blocked by the ORB self-host error-tracking sub-issue (PostHog error tracking for the ORB self-host server (swap the src/selfhost/sentry.ts sink; cron-monitor heartbeat design) #8287) — reuses its PostHog client + redaction plumbing.

Current state

The ORB's AI calls flow through src/selfhost/ai.ts (provider attempts, rate limits, close-breaker engagement — the ai subsystem in the existing Sentry taxonomy), feeding advisory routing and per-reviewer AI votes. Cost/latency visibility today is logs + whatever Sentry catches on failure; there is no per-model spend/latency analytics at all.

Scope

• Spike first: verify raw $ai_generation capture shape against a real PostHog project (event properties: model, provider, latency, token counts, cost where derivable, error/rate-limit outcome) — metagraphed's spike validates the Workers path; ours validates posthog-node from the self-host server.
• Wire capture at the src/selfhost/ai.ts chokepoint (attempt-level: one event per provider attempt, including failed/rate-limited attempts — the retry/breaker behavior is exactly what needs visibility).
• Prompt/response content is never captured. Metadata only (the existing PAYLOAD_KEY scrub treats prompts as payload — same rule here; $ai_generation's optional content fields stay empty by policy).
• Dashboard: spend/latency/failure by provider+model+effort, wired into the insights sub-issue's home.
• Tests: capture fires per attempt with metadata-only properties; unconfigured no-op.

Boundaries

• Miner-side AMS AI calls are out of scope for the first pass (different package, different client) — noted for a follow-up once the ORB pattern is proven.

Resources

• https://posthog.com/docs/ai-engineering/observability
• Spike: AI observability — verify raw $ai_generation capture, wire AI-backed routes metagraphed#7763

[linear-code]

JSO-5741