fix(agent-presence): upsert atomically — kill first-time-insert race

Previous setStatus() did read-modify-write:
  findOne → if-exists save / else create+save

Two concurrent first-time writes for the same userId both saw no row,
both INSERT'd, second hit unique-key (agent_presences.PRIMARY) and 500'd
with "Duplicate entry '<userId>' for key 'agent_presences.PRIMARY'" —
visible in prod (2026-05-25 23:23:35Z) when Fabric.OpenclawPlugin's
presence-sync emitted two PUTs ~10 ms apart for the same agent (its
tick-overlap is being fixed separately in nav/Fabric.OpenclawPlugin).

Replace with repo.upsert(values, ['userId']) — compiles to MySQL
`INSERT … ON DUPLICATE KEY UPDATE`, atomic at the storage engine,
no read needed, no race window. Synthesize the returned entity from
the values we just wrote rather than a SELECT round-trip; controller
only reads {userId, status} off it.

Sim verified with 5 parallel PUTs to a fresh userId: all 200, no
Duplicate errors in guild log (was: 1 × 200 + 4 × 500 with the
old code).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/agents/agent-presence.service.ts

@@ -39,15 +39,23 @@ export class AgentPresenceService {
    * Upsert a user's presence. Source is a free-text tag for debugging
    * (e.g. "hf-plugin", "manual", "test"). PUT /agents/:id/presence
    * calls this; the plugin pushes only on diff so writes are sparse.
+   *
+   * Implementation note: the older findOne+save split was a read-modify-
+   * write race — two concurrent first-time writes for the same userId
+   * would both read no row, both INSERT, second hits unique-key dup
+   * (`agent_presences.PRIMARY`) and 500s. Fabric.OpenclawPlugin's
+   * presence-sync occasionally fires two PUTs for the same agent within
+   * ~10 ms (tick overlap on its side — separate fix in the plugin),
+   * which surfaced this race in prod.
+   *
+   * `repo.upsert(values, conflictPaths)` compiles to MySQL
+   * `INSERT … ON DUPLICATE KEY UPDATE` and is atomic at the storage
+   * engine level — no read needed, no race window. We synthesize the
+   * returned entity from what we just wrote rather than round-tripping
+   * a SELECT — the controller only reads {userId, status} off it.
    */
   async setStatus(userId: string, status: PresenceStatus, source: string): Promise<AgentPresence> {
-    const existing = await this.repo.findOne({ where: { userId } });
-    if (existing) {
-      existing.status = status;
-      existing.source = source;
-      return this.repo.save(existing);
-    }
-    const row = this.repo.create({ userId, status, source });
-    return this.repo.save(row);
+    await this.repo.upsert({ userId, status, source }, ['userId']);
+    return this.repo.create({ userId, status, source });
   }
 }