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/agents-crud.ts

@@ -295,12 +295,18 @@ export function buildAgentCrudMethods(
       if (agents.length === 0) return [];
 
       const lowerText = text.toLowerCase();
-      const aliasMap = new Map<string, string[]>();
+      // Per-alias agent set. Storing a `Set` (not an array) means the
+      // same agent contributing the same string via multiple fields
+      // (e.g. name === role === "CEO") collapses to one entry. Before
+      // this fix the duplicate push made the alias look non-unique, the
+      // unique-alias filter dropped it, and every @mention in a scaffold
+      // where role mirrored name silently missed.
+      const aliasMap = new Map<string, Set<string>>();
       const registerAlias = (alias: string, agentId: string) => {
         const normalized = alias.trim().toLowerCase();
         if (!normalized) return;
-        const existing = aliasMap.get(normalized) ?? [];
-        existing.push(agentId);
+        const existing = aliasMap.get(normalized) ?? new Set<string>();
+        existing.add(agentId);
         aliasMap.set(normalized, existing);
       };
       for (const agent of agents) {
@@ -310,8 +316,12 @@ export function buildAgentCrudMethods(
       }
 
       const matched = new Set<string>();
+      // Only aliases that unambiguously resolve to a single agent are
+      // treated as mentions. Two agents sharing a role ("CEO" appearing
+      // twice in the company) make `@ceo` ambiguous and we refuse to
+      // pick one silently.
       const uniqueAliases = [...aliasMap.entries()]
-        .filter(([, ids]) => ids.length === 1)
+        .filter(([, ids]) => ids.size === 1)
         .sort((a, b) => b[0].length - a[0].length);
       for (const [alias, ids] of uniqueAliases) {
         const pattern = `@${alias}`;
@@ -319,7 +329,8 @@ export function buildAgentCrudMethods(
         if (idx === -1) continue;
         const afterIdx = idx + pattern.length;
         if (afterIdx >= lowerText.length || /[\s,!?.;:\])}>]/.test(lowerText[afterIdx]!)) {
-          matched.add(ids[0]!);
+          const [onlyId] = ids;
+          if (onlyId) matched.add(onlyId);
         }
       }
       return [...matched];