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/shared/contracts/channels.ts

@@ -92,6 +92,7 @@ export const IPC_CHANNELS = {
   getThread: "agent-company:get-thread",
   listThreadEvents: "agent-company:list-thread-events",
   listThreadWaitStates: "agent-company:list-thread-wait-states",
+  listThreadParticipants: "agent-company:list-thread-participants",
   windowMinimize: "window:minimize",
   windowMaximize: "window:maximize",
   windowClose: "window:close",

src/shared/contracts/desktop-api/threads.ts

@@ -1,10 +1,11 @@
 import type { z } from "zod";
-import type { ConversationEventRecord, ConversationThreadRecord, ThreadWaitStateRecord } from "../../types";
-import type { listThreadEventsSchema, listThreadWaitStatesSchema, threadGetterSchema } from "../schemas-threads";
+import type { ConversationEventRecord, ConversationParticipantRecord, ConversationThreadRecord, ThreadWaitStateRecord } from "../../types";
+import type { listThreadEventsSchema, listThreadParticipantsSchema, listThreadWaitStatesSchema, threadGetterSchema } from "../schemas-threads";
 import type { DesktopResult } from "./common";
 
 export type ThreadsApi = {
   getThread: (input: z.infer<typeof threadGetterSchema>) => Promise<DesktopResult<ConversationThreadRecord>>;
   listThreadEvents: (input: z.infer<typeof listThreadEventsSchema>) => Promise<DesktopResult<ConversationEventRecord[]>>;
   listThreadWaitStates: (input: z.infer<typeof listThreadWaitStatesSchema>) => Promise<DesktopResult<ThreadWaitStateRecord[]>>;
+  listThreadParticipants: (input: z.infer<typeof listThreadParticipantsSchema>) => Promise<DesktopResult<ConversationParticipantRecord[]>>;
 };

src/shared/contracts/schemas-threads.ts

@@ -16,3 +16,8 @@ export const listThreadWaitStatesSchema = z.object({
   companyId: z.string().uuid(),
   resolvedOnly: z.boolean().default(false),
 });
+
+export const listThreadParticipantsSchema = z.object({
+  companyId: z.string().uuid(),
+  threadId: z.string().uuid(),
+});