pin: bump Worldtree spec to f1b59f8 (v0.35.16) — cold recall closes end-to-end

Worldtree shipped its half of the union-recall fix: #297 (client-side
per-scope-value union recall) + #298/#299 (adopt the bifrost v0.6
scope_any/scope_all wire, v0.35.16). It now emits scope_any on the recall
path, pairing with our v0.17.6 provider — cold cross-session recall is
closed end-to-end (pending a live re-smoke against a v0.35.16 instance).

Re-vendored conversation-api-spec.md + conversation_api.contract.md;
285-commit catch-up (v0.29.0 -> v0.35.16). Diff-reviewed: no client-facing
breaking changes for our consumer.

- #211 agent-slug rename (saga->echo, actor->mask) — slugs only, we pass --agent
- #245 end_user_id persistence + memory-scope resolver (additive)
- #187/#188/#219 Tier-3 define/PATCH policy (additive); error codes stable
- bifrost binding field + ephemeral_does_not_accept_bifrost 422 now documented (#17 surface)
- docs: SPEC-PIN.md pin table + history; bifrost-self-test recall status; persistent-memory

No package version bump (docs/pin-only, no ratatoskr code change).

docs/bifrost-self-test.md

@@ -155,8 +155,10 @@ Branch (a), scope asymmetry — fed to #297.
 into `scope_all` (AND) + `scope_any` (OR/union). Worldtree can now send the visible
 scopes as a `scope_any` union (e.g. `[{end_user: smoke-user}, {end_user: smoke-user,
 agent_self: ...}]`), so the subset-scoped chunk recalls via the matching OR member.
-Our store implements this at parity with the v0.6 reference; **end-to-end cold recall
-now waits only on Worldtree emitting `scope_any`** on the recall path (#297).
+Our store implements this at parity with the v0.6 reference; Worldtree **adopted the
+v0.6 wire and now emits `scope_any`** on the recall path (#297 client-side union recall
++ #298/#299 bifrost-v0.6 adoption, v0.35.16), so cold cross-session recall is **closed
+end-to-end** — pending a live re-smoke against a personal instance running v0.35.16.
 
 ## Notes / foot-guns