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/lib/agentTools.ts

@@ -107,7 +107,7 @@ const TOOLS: ToolDef[] = [
       };
       const recipient = Object.entries(directory).find(([k]) => target.includes(k))?.[1];
       if (!recipient) return { ok: false, display: `I don't know who "${target}" is. Try Mariana, Aisha, or Rohan.` };
-      const threads = await chatApi.listThreads();
+      const threads = await chatApi.listThreads(ctx.userEmail);
       const existing = threads.find((t) => t.participants.includes(recipient) && t.participants.includes(ctx.userEmail));
       const threadKey = existing?._key || (await chatApi.createThread(`${ctx.userEmail} ↔ ${recipient}`, [ctx.userEmail, recipient]));
       await chatApi.sendMessage(threadKey, body, ctx.userEmail);