feat(agent): GPU load readout + live worker-count tuning (#114)

Control UI gains what the operator asked for:
- GPU load (nvidia-smi): util %, VRAM used/total + bar, temp — so you can see how
  hard the card is working while you're at the desktop.
- Worker count is now a live − / + control (POST /concurrency), not just an env:
  the worker is a pool of independent slots (shared model, so slots add concurrent
  inference, not N× VRAM). Dial up for speed, down to free the card. Replaces
  pause/resume with Start/Stop + the worker dial.
- Graceful release on stop / pool-shrink: a slot hands its still-leased jobs back
  via client.release() so they're re-picked immediately (pairs with the server
  recovery sweep).

Not CI-tested (agent/ outside CI) — verified by running.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

agent/fc_agent/client.py

@@ -54,6 +54,19 @@ class FcClient:
         except requests.RequestException:
             pass
 
+    def release(self, job_ids: list[int]) -> None:
+        # Graceful hand-back on stop so orphaned work is re-leased at once.
+        if not job_ids:
+            return
+        try:
+            self.s.post(
+                f"{self.base}/api/gpu/jobs/release",
+                json={"agent_id": self.agent_id, "job_ids": job_ids},
+                timeout=30,
+            )
+        except requests.RequestException:
+            pass
+
     def fetch_image(self, image_url: str) -> bytes:
         # image_url is a server-relative path ("/images/...").
         r = self.s.get(f"{self.base}{image_url}", timeout=180)