feat(agents): implement state-aware failover and lane suspension

Summary:
- Persist quota-suspension state transitions and reload fresh suspension state before failover handoff injection.
- Restore suspended lanes to configured concurrency and share failover-to-suspension reason mapping across fallback and embedded runner paths.
- Export model.failover diagnostics via OTLP and cover queueing/resume behavior with regressions.

Verification:
- pnpm test src/config/sessions/store.pruning.integration.test.ts src/process/command-queue.test.ts src/agents/session-suspension.test.ts src/agents/model-fallback.test.ts extensions/diagnostics-otel/src/service.test.ts
- git diff --check
- pnpm exec oxfmt --check --threads=1 on changed TypeScript files
- GitHub checks: 92 successful, 0 pending, 0 failed on head 962146be88
- Review threads: none unresolved

src/process/command-queue.test.ts

@@ -364,6 +364,35 @@ describe("command queue", () => {
     }
   });
 
+  it("keeps work queued while a lane has zero concurrency and drains after resume", async () => {
+    const lane = `suspended-lane-${Date.now()}-${Math.random().toString(16).slice(2)}`;
+    setCommandLaneConcurrency(lane, 0);
+
+    let ran = false;
+    const task = enqueueCommandInLane(lane, async () => {
+      ran = true;
+      return "resumed";
+    });
+
+    await Promise.resolve();
+    expect(ran).toBe(false);
+    expect(getCommandLaneSnapshot(lane)).toMatchObject({
+      activeCount: 0,
+      queuedCount: 1,
+      maxConcurrent: 0,
+    });
+
+    setCommandLaneConcurrency(lane, 1);
+
+    await expect(task).resolves.toBe("resumed");
+    expect(ran).toBe(true);
+    expect(getCommandLaneSnapshot(lane)).toMatchObject({
+      activeCount: 0,
+      queuedCount: 0,
+      maxConcurrent: 1,
+    });
+  });
+
   it("getCommandLaneSnapshot reports active and queued work for one lane", async () => {
     const lane = `snapshot-lane-${Date.now()}-${Math.random().toString(16).slice(2)}`;
     setCommandLaneConcurrency(lane, 1);