refactor(agent): DRY pass on the GPU agent worker package

Consolidate genuine duplication in agent/fc_agent into single-source
helpers (behavior-preserving; DRY Pass process #594):

worker.py
- _fail(jid, image_id, exc, verb) — 4 terminal "fail this job" blocks
  (downloader HTTP-fault + decode, consumer non-transient + generic).
- _release(job_ids) (was _release_owned) — the one lease hand-back path;
  6 inline release([jid])+unhold sites now route through it.
- _stopped(stop_evt) + _abort_if_stopped(jid, stop_evt) — 4 stop-check
  -and-release blocks and every bare stop-check.
- _timed(stage) contextmanager — ~8 monotonic()/_record() timing pairs;
  records only on clean exit, matching the old skip-on-raise behavior.
- _ewma(prev, x, alpha) module fn — 3 EWMA updates in the autoscaler.

client.py
- _submit(path, payload) — submit / submit_embedding (retrying session).
- _post_quiet(path, payload) — heartbeat / fail / release fire-and-forget.

detectors.py
- Proposers._top(detector, image, cap) — merges components() and panels().

config.py
- _bool_env(name, default) — auto_start / auto_scale env parsing.

Left alone (recorded): the xyxy→norm-xywh conversion duplicated across
models.py/detectors.py (2 copies, independent wrapper modules — sharing
would couple them), and the _ensure_embedder/_ensure_proposers pair (same
lock shape, different concepts).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/fc_agent/config.py

@@ -8,6 +8,11 @@ import os
 from dataclasses import dataclass
 
 
+def _bool_env(name: str, default: str = "") -> bool:
+    """A boolean env var — present + truthy ('1'/'true'/'yes') → True."""
+    return os.environ.get(name, default).lower() in ("1", "true", "yes")
+
+
 @dataclass
 class Config:
     fc_url: str          # base URL of the FabledCurator web service
@@ -57,8 +62,8 @@ class Config:
             poll_idle_seconds=float(os.environ.get("POLL_IDLE_SECONDS", "10")),
             embed_dtype=os.environ.get("SIGLIP_DTYPE", "float16"),
             embed_model_override=os.environ.get("EMBED_MODEL_NAME", ""),
-            auto_start=os.environ.get("AUTO_START", "").lower() in ("1", "true", "yes"),
-            auto_scale=os.environ.get("AUTO_SCALE", "true").lower() in ("1", "true", "yes"),
+            auto_start=_bool_env("AUTO_START"),
+            auto_scale=_bool_env("AUTO_SCALE", "true"),
             person_weights=os.environ.get("PERSON_WEIGHTS", "yolo11n.pt"),
             person_conf=float(os.environ.get("PERSON_CONF", "0.35")),
             anatomy_weights=os.environ.get("ANATOMY_WEIGHTS", ""),