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/gpu.py

@@ -0,0 +1,30 @@
+"""GPU load readout via nvidia-smi (present in the container thanks to the
+NVIDIA Container Toolkit's `utility` capability). Returns None if unavailable —
+the UI just shows n/a (e.g. CPU-fallback run)."""
+import subprocess
+
+
+def read_gpu() -> dict | None:
+    try:
+        out = subprocess.run(
+            [
+                "nvidia-smi",
+                "--query-gpu=utilization.gpu,memory.used,memory.total,temperature.gpu",
+                "--format=csv,noheader,nounits",
+            ],
+            capture_output=True, text=True, timeout=5, check=True,
+        ).stdout.strip().splitlines()
+    except (OSError, subprocess.SubprocessError):
+        return None
+    if not out:
+        return None
+    parts = [p.strip() for p in out[0].split(",")]
+    try:
+        return {
+            "util_pct": int(float(parts[0])),
+            "mem_used_mb": int(float(parts[1])),
+            "mem_total_mb": int(float(parts[2])),
+            "temp_c": int(float(parts[3])),
+        }
+    except (ValueError, IndexError):
+        return None