refactor: retire the KB-recall bridge — WT #383 native reference_knowledge (b167)

Worldtree #383 shipped native Tier-3 reference_knowledge (v1.0.0b167, live on :8081 +
demo): every Tier-3 agent context now carries the tool automatically, with evidence
packets (note_id + path provenance, confidence bucket) and a server-side grounding
rule. That supersedes the interim consumer-side memory_context pinning bridge (slice
3), so it is deleted per its INV-KB-1 retire seam.

Removed:
- src/ratatoskr/kb_bridge.py + tests/test_kb_bridge.py (the whole module).
- server.py: the pin_kb_context import + the single turn-path call-site (reverted to
  the pre-bridge wt.stream_turn call), the SSE keepalive that only covered the consult
  delay, and the bridge-only agent_id plumbing (TurnHandle.agent_id + the submit read).
- index.html: agent_id dropped from the turn POST body.
- test_web_server.py: TestKbBridgeWiring (tested the removed call-site).

Kept:
- wt.stream_turn's memory_context param (inert SDK-parity passthrough; worldtree-dev
  concurred it stays) + its forwarding tests.
- the non-str content 400 guard (general input hygiene, not bridge-specific).

Retirement LIVE-VERIFIED before deletion: a Donut session on :8081/b167 carries
builtin_tools=['reference_knowledge']; she called it and grounded in the DCC Collapse
content fully in-voice, degrading gracefully on absent content. 524 green.

Contract marks slice-3 RETIRED (historical record retained). #383 closed.

docs/contracts/donut_voiced_interview.contract.md

@@ -40,6 +40,19 @@ confidence: 0.8
 
 # Contract: Donut voiced interview (auto-TTS + KB-recall bridge)
 
+> **⚠ SLICE 3 (KB-recall bridge) RETIRED 2026-08-02.** The `kb_bridge.py` module +
+> its single `web/server.py` call-site were deleted per INV-KB-1 when Worldtree #383
+> shipped native Tier-3 `reference_knowledge` (v1.0.0b167, live on :8081 + demo).
+> Donut now searches the fiction wing (DCC corpus) natively, in-turn, with evidence
+> packets (note_id + path provenance, confidence bucket) and a server-side grounding
+> rule — strictly better than the consumer-side memory_context pinning it replaced
+> (no separate consult round-trip, not gated by our out-of-band prompt). Retirement
+> live-verified: Donut called `reference_knowledge` and grounded in DCC in-voice
+> before deletion. `wt.stream_turn`'s `memory_context` param was KEPT (inert SDK
+> parity). The DEC-6 / INV-KB-* / FN pin_kb_context sections below are retained as
+> historical record of what was built and why it retired. **Slices 1 (persona) + 2
+> (auto-TTS) remain LIVE.**
+
 Migration-style contract: three separable slices (persona / TTS / KB-bridge),
 each independently shippable. Slice order is chosen for fastest visible result.