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.ts

@@ -14,7 +14,7 @@ export class AppDatabase extends AppDatabaseBase {
 
   // ─── Messages ──────────────────────────────────────────────────────────────
 
-  sendAgentMessage(input: { companyId: string; fromAgentId: string; toAgentId?: string | null; channel?: string; channelTargetId?: string | null; subject: string; body: string; priority?: string; parentMessageId?: string | null; attachmentsJson?: string; senderKind?: "agent" | "human" | "system" }) { return this.messages.sendAgentMessage(input); }
+  sendAgentMessage(input: { companyId: string; fromAgentId: string; toAgentId?: string | null; channel?: string; channelTargetId?: string | null; subject: string; body: string; priority?: string; parentMessageId?: string | null; attachmentsJson?: string; senderKind?: "agent" | "human" | "system"; mentionedAgentIds?: readonly string[] }) { return this.messages.sendAgentMessage(input); }
   getAgentMessage(messageId: string, companyId: string) { return this.messages.getAgentMessage(messageId, companyId); }
   listAgentMessages(companyId: string, opts?: { agentId?: string; messageId?: string; channel?: string; channelTargetId?: string | null; unreadOnly?: boolean; limit?: number }) { return this.messages.listAgentMessages(companyId, opts); }
   searchMessages(companyId: string, query: string, opts?: { channel?: string; channelTargetId?: string | null; limit?: number }) { return this.messages.searchMessages(companyId, query, opts); }