Make @mentions visible to the agents they name

A DM addressed to one agent that @-mentions another was silently
dropping the second agent out of the conversation:

* `dispatchAgentMessage` added the mentioned agent to the wake set,
  so CEO's heartbeat fired — but
* `listAgentInboxUnread` only surfaced messages where the current
  agent was `to_agent_id` or sat in a matching channel/project/
  department. Mentions were invisible, so CEO's next turn saw nothing
  new and returned silently. From the operator's chair this looked
  exactly like "CEO ignored me; 张工程师 answered instead".
* `findMentionedAgentIds` had a latent bug on top: its unique-alias
  filter used `ids.length === 1`, but a single agent whose name
  matched its role registered itself twice, tripped the filter, and
  every `@CEO` in a scaffold with role === name was a no-op. Once
  inbox visibility was fixed the bug became load-bearing.

Root-cause fixes:

* New migration v48 adds `agent_message_mentions(message_id, agent_id,
  company_id, created_at)` so mention fan-out is a first-class join.
* `sendAgentMessage` now takes the resolved mention set and writes
  the row + mentions in a single transaction — a crash between the
  two writes can no longer leave a message visible to the recipient
  but invisible to the mentioned agent.
* `dispatchAgentMessage` computes the mention set exactly once via
  `resolveMentionedAgentIds` and hands the same list to both the DB
  write and `resolveMessageWakeTargets`, so persistence and wake
  stay in lockstep.
* `listAgentInboxUnread` and `listAgentMessages` gain an `id IN
  (select message_id from agent_message_mentions …)` clause so the
  mentioned agent's inbox actually contains the message that woke it.
* `findMentionedAgentIds` now stores per-alias agent IDs in a `Set`
  and filters on `ids.size === 1`, dropping the self-duplication
  bug while preserving the "refuse ambiguous aliases" semantics.
* Company teardown drops `agent_message_mentions` alongside the
  existing `agent_message_reads` cleanup so deletes stay clean.

Tests pin the new behaviour: a DM with a `@CEO` mention reaches
CEO's inbox and both agents get woken, while a DM whose recipient
is `__board__` never attempts to wake the human sentinel.

src/main/database/migrations/v48-v48.ts

@@ -0,0 +1,46 @@
+import log from "electron-log/main.js";
+import type { Migration, MigrationDeps } from "./types";
+
+export function createMigrationsV48V48(deps: MigrationDeps): Migration[] {
+  const { db } = deps;
+  return [
+    {
+      // Persist @mention fan-out so inbox visibility stops depending on
+      // a runtime-only regex against every message body.
+      //
+      // Before v48 `dispatchAgentMessage` woke mentioned agents via
+      // `findMentionedAgentIds`, but `listAgentInboxUnread` filtered
+      // purely on `channel`, `to_agent_id`, and department/project
+      // membership. A mid-chat `@ceo update please` targeted at
+      // 张工程师 therefore woke CEO with an empty inbox — CEO's
+      // heartbeat fired, saw nothing new, and stayed silent while the
+      // DM recipient replied and the operator assumed CEO had ignored
+      // them. The join table makes "mentioned in a message" a first-
+      // class visibility path: persisted once at dispatch, joined on
+      // read, so every consumer (inbox projection, read-receipts,
+      // search) sees a single source of truth.
+      version: 48,
+      name: "agent_message_mentions",
+      fn: () => {
+        db.exec(`
+          CREATE TABLE IF NOT EXISTS agent_message_mentions (
+            message_id TEXT NOT NULL,
+            agent_id TEXT NOT NULL,
+            company_id TEXT NOT NULL,
+            created_at TEXT NOT NULL,
+            PRIMARY KEY (message_id, agent_id)
+          )
+        `);
+        db.exec(`
+          CREATE INDEX IF NOT EXISTS idx_agent_message_mentions_agent
+          ON agent_message_mentions(company_id, agent_id, created_at)
+        `);
+        db.exec(`
+          CREATE INDEX IF NOT EXISTS idx_agent_message_mentions_message
+          ON agent_message_mentions(message_id)
+        `);
+        log.info("[migration v48] agent_message_mentions join table installed");
+      },
+    },
+  ];
+}