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/config.py

@@ -8,7 +8,8 @@ class Config:
     fc_url: str          # base URL of the FabledCurator web service
     token: str           # the bearer token from Settings → Tagging → GPU agent
     agent_id: str        # identifies this agent's leases
-    batch_size: int      # jobs leased per round (concurrency is still 1)
+    batch_size: int      # jobs a worker leases per round
+    concurrency: int     # INITIAL parallel workers (tunable live from the UI)
     ccip_model: str      # imgutils CCIP model name ("" → imgutils default)
     detector_level: str  # imgutils person-detector level: n|s|m|x
     poll_idle_seconds: float   # wait between empty leases
@@ -20,6 +21,7 @@ class Config:
             token=os.environ.get("FC_TOKEN", ""),
             agent_id=os.environ.get("AGENT_ID", "desktop-agent"),
             batch_size=int(os.environ.get("BATCH_SIZE", "4")),
+            concurrency=int(os.environ.get("CONCURRENCY", "1")),
             ccip_model=os.environ.get("CCIP_MODEL", ""),
             detector_level=os.environ.get("DETECTOR_LEVEL", "m"),
             poll_idle_seconds=float(os.environ.get("POLL_IDLE_SECONDS", "10")),