feat: phase 5 of AI pipeline hardening — R42 retrieval & telemetry

- Add dependency-free TF-IDF retrieval (src/lib/retrieval.js) with NL+EN
  stopwords and a WeakMap-cached index.
- Rewrite buildKbContext to ship the top-K relevant topics + verbatim-
  mentioned ids only, filter relations to the included set, and append a
  [kb_hash: <8 hex>] suffix so the ephemeral prompt cache busts when the
  graph changes. Returns { context, retrievedTopics, allTopics }.
- Add LOOKUP_TOPIC_TOOL and drive useChat through callLLM directly with a
  multi-hop tool_result loop capped at 3 hops; preserve Anthropic-provided
  tool_use ids through callLLM so the loop can echo correct tool_use_id.
- Truncate R42 history to the last 12 turns and prepend a single
  "(earlier conversation truncated)" assistant message.
- Set R42 chat defaults: temperature 0.3, maxTokens 2048.
- Add pb_migrations/1780500002_created_llm_calls.js (the best-effort
  logger in callLLM was already wired) and a new Admin → Diagnostics
  view showing the last 100 calls with token usage, cache-hit rate, and
  USD cost from a local Anthropic price table.
- Finalize AI_PIPELINE_HARDENING_PLAN.md: mark Phases 1–5 shipped and
  Phase 6 (eval harness) explicitly out of scope.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

AI_PIPELINE_HARDENING_PLAN.md

@@ -6,6 +6,8 @@
 
 This plan upgrades the platform's interaction with the Anthropic API: how prompts are built, how responses are parsed, how the model is retried, and how outputs are validated. It is broken into six phases that can be implemented and shipped independently. Each phase ends with verifiable acceptance criteria.
 
+> **Status (2026-05-20):** Phases 1–5 implemented and shipped. Phase 6 (eval harness) is intentionally **out of scope** for this initiative — the production pipeline is hardened to the level the platform needs, and a golden-set runner can be reopened later as a stand-alone task if regression risk grows. The repo no longer carries any TODO from this plan.
+
 ---
 
 ## 0. Operating principles
@@ -461,7 +463,9 @@ Wire `callLLM` to write to it (best-effort, never throws). Add a minimal `Admin
 
 ---
 
-## Phase 6 — Eval harness (optional, high-leverage)
+## Phase 6 — Eval harness (NOT IMPLEMENTED — out of scope)
+
+> Phase 6 was deliberately skipped. The acceptance criteria for Phases 1–5 give enough confidence in extraction, content, quiz, R42, and telemetry that a golden-set harness is not currently load-bearing. Leave this section intact as a starting point for a future, separately-scoped initiative.
 
 **Goal:** prompt or model changes can be measured before they ship.