feat(chat): per-user inbox anchor for listing kind=value threads

EA2 doesn't expose a generic flow-by-kind list endpoint; flow/processes
is definition-only. Migrated chat to flow.kind=value, so direct listing
needed a new strategy.

- Each user has a deterministic inbox flow.kind=value doc keyed by
  email slug. ensureInbox() creates it idempotently on first listThreads.
- createThread links the new thread into BOTH participants' inboxes via
  defines edges with role='presentation' (each link is one apply-batch).
- listThreads(userEmail) reads defines edges from the user's inbox key.
  No more reliance on source_context regex filtering through the
  definition catalogue.

Updated all 3 call sites: Chat.tsx (mount + after-create refresh) and
agentTools.send_chat. 31/31 tests green.

src/scenes/Chat.tsx

@@ -34,7 +34,7 @@ export default function Chat() {
     let cancelled = false;
     setLoadingThreads(true);
     chatApi
-      .listThreads()
+      .listThreads(me)
       .then((rows) => {
         if (cancelled) return;
         setThreads(rows);
@@ -73,7 +73,7 @@ export default function Chat() {
     try {
       const otherLabel = personaLabel(recipient);
       const key = await chatApi.createThread(`${personaLabel(me)} ↔ ${otherLabel}`, [me, recipient]);
-      const next = await chatApi.listThreads();
+      const next = await chatApi.listThreads(me);
       setThreads(next);
       setActiveKey(key);
       pushToast("ok", `Thread opened with ${otherLabel}`);