fix(thread-panel): wire the scoped reply composer to real participants

The thread-panel composer was pinned to `channel="direct"` with no
`directParticipantIds` and no `replyTo`. Clicking reply in a human
review thread sent a message with no recipient and the send button
stayed disabled — a silent dead-end for the entire Co-Founder
review UX.

The fix exposes the thread's participant list to the renderer:

  - `listThreadParticipantsSchema` and the matching IPC channel
    (`agent-company:list-thread-participants`) return the rows from
    `conversation_participants` for a given thread.
  - `threadIntegration.onTaskCreated` / `onApprovalCreated` already
    enrol the owning agent and the virtual user on thread creation,
    so the list is populated without any backfill.
  - `ThreadPanel` loads participants alongside events and forwards
    the agent ids to `MessageComposer.directParticipantIds`. The
    composer's existing `directRecipientId` memo picks the first
    non-board agent, so replies now target the correct agent — or
    surface the "no recipient" notice when the thread has none.

The Board/user member is deliberately excluded from the list the
composer consumes: the composer wants an agent to route to. All
other wiring (wait-state precision, threadId) stays untouched.

src/main/ipc-thread-handlers.ts

@@ -1,6 +1,7 @@
 import {
   IPC_CHANNELS,
   listThreadEventsSchema,
+  listThreadParticipantsSchema,
   listThreadWaitStatesSchema,
   threadGetterSchema,
 } from "@shared/contracts";
@@ -35,4 +36,14 @@ export function registerThreadHandlers({ registerHandle, db, ensureCompanyExists
     const waitStates = db.listThreadWaitStates(input.companyId, { resolvedOnly: input.resolvedOnly });
     return ok(waitStates);
   });
+
+  registerHandle(IPC_CHANNELS.listThreadParticipants, async (_event, payload) => {
+    const input = listThreadParticipantsSchema.parse(payload);
+    const companyError = ensureCompanyExists(input.companyId);
+    if (companyError) return companyError;
+    const thread = db.getThread(input.threadId, input.companyId);
+    if (!thread) return fail("THREAD_NOT_FOUND", "Thread not found");
+    const participants = db.listParticipants(input.threadId);
+    return ok(participants);
+  });
 }