feat(chat): migrate from flow.kind=definition hack to flow.kind=value

Oracle deferred caveat #2 — chat-on-flow-collection hack — CLOSED.

Previous design used flow.kind=definition + source_context=EA2_CHAT_THREAD
for chat threads and EA2_CHAT_MSG for messages, then filtered them out of
hub/wizard surfaces via a NON_BUSINESS_SOURCE_CONTEXTS exclusion. That
was containment, not a clean model.

Verified EA2 contract: flow.kind='value' is a valid enum value (probed
against ea2.baseline.svc — kind=value returns 200, kind=conversation/
thread/message/channel/note all return 400 schema violation).

- Thread: flow.kind=value, source_context=CANVAS_CHAT_THREAD
- Message: flow.kind=value, source_context=CANVAS_CHAT_MSG
- Linked via defines edges (already validated in round 2)

Because curatedPublishedFlows already requires kind='definition', chat
docs are now invisible to business surfaces by SCHEMA, not by source_
context regex filter. The filter is kept as defence-in-depth (+ legacy
docs created during round 2 still need to be filtered out).

Tried data collection first; it requires relates(role=sourced_from) edge
to an sdx_connections handle per the P19 invariant — wrong model for
chat, so flow.kind=value is the right home.

src/lib/flowCuration.ts

@@ -31,6 +31,8 @@ const DEV_ARTEFACT_PATTERNS = [
 const NON_BUSINESS_SOURCE_CONTEXTS = new Set([
   "EA2_CHAT_THREAD",
   "EA2_CHAT_MSG",
+  "CANVAS_CHAT_THREAD",
+  "CANVAS_CHAT_MSG",
   "EA2_WIZARD_STEP",
   "EA2_DRAFT_PROCESS:process_creation",
 ]);