dbt PR review: CI experience hides the engine's proofs (undocumented artifacts, misleading lint-only banner, silent AI lane, guessed base ref)

## Summary

Running the dbt PR review end to end (altimate-code 0.10.0 on `jaffle_shop_duckdb` with five injected changes) and mining the 30 most recent reviews in our own dogfood repo surfaced a cluster of CI-experience problems. None change what the engine proves; all change whether users see it.

## Problems

1. **Artifacts that decide fidelity are undocumented.** Equivalence and lineage-breakage proofs need `target/catalog.json` (`dbt docs generate`) and compiled SQL for the base ref in `target-base/compiled`. Neither appears in the usage doc quick-start, `github/review/action.yml`, or `github/review/examples/altimate-ingestion.yml`; `target-base` is mentioned only in prose. 57% of our dogfood reviews ran lint-only.
2. **"Lint-only run — no dbt manifest/warehouse was available" is shown when a manifest was present.** `summary.degraded` is `runDegraded || findings.some(f => f.degraded)`, so a single undecidable finding flips the whole-run banner (`orchestrate.ts`, `format.ts`).
3. **The AI lane fails silently.** Every failure path in `runAiReview` returns `[]` with only a log; telemetry shows 18% of CI runs call `review_ai_prompt` and never `review_ai_parse`. A default run with no credentials attempted a stale local endpoint every time; the docs say the lane is "skipped".
4. **Base ref is guessed.** `defaultBaseRef` walks `origin/main → origin/master → main → master` and ignores the event's `pull_request.base.ref`. A PR against `deployment` was diffed against `main`.
5. **One rule is half of all findings.** "new model has no uniqueness/grain test" was 427 of 853 findings across 30 dogfood PRs, one bullet per model, never acted on.
6. **Headless review telemetry has no `project_id`.** `Telemetry.setContext` is only called from the interactive session loop, so `altimate review` events cannot be grouped by repo.
7. **PR title/body never reach the AI lane in CI**, so its intent check cannot run.
8. Docs describe PII classification as "columns that newly expose sensitive data"; the lane flags any PII column present in a touched model.

## Fix (this PR)

Separate `lintOnly` from `undecidableFindings` in the envelope and render each honestly; add artifact hints with the exact commands; AI lane returns `ok | skipped | timeout | error` with a reason, rendered in the summary and recorded in `review_run` (`ai_status`, `ai_findings`, `undecidable_findings`); scale the AI timeout with prompt size; read base ref and PR title/body from `GITHUB_EVENT_PATH`; group repeated grain-test findings in the summary while keeping atomic findings; set `project_id` on the headless path; update the docs, action and example workflow to compile head and base and generate the catalog.

## Out of scope, tracked in `docs/internal/2026-09-03-dbt-pr-review-deep-dive.md`

Verdict provenance (catalog rules can currently block, contrary to the README), loading `.altimate/review.yml` from the base rather than the PR head, gate lifecycle / override wiring, inline-comment dedupe, positional equivalence false positive (core), feedback loop.

Start with the end-to-end dbt PR review on jaffle_shop_duckdb, then trace summary.degraded in orchestrate.ts and format.ts and the failure paths in runAiReview. Read github/review/action.yml and github/review/examples/altimate-ingestion.yml alongside the GITHUB_EVENT_PATH path. Done means the listed CI statuses, artifact hints, metadata, telemetry fields, grouped findings, and documentation are represented honestly.

Written by the indexing model from the issue text.