Add describe (first-pass) mode to the judge node

New mode on QwenVLImageJudge: 'describe' looks at the reference alone and returns
a prompt-ready caption + per-axis target spec to seed the very first prompt (the
generator has nothing to reproduce yet). 'compare' is the existing ref-vs-gen
scoring. generated_image is now optional (required only for compare); shared
generation refactored into _generate_from_messages; third output renamed
diff_analysis -> analysis (mode-agnostic). agent_bridge gains --mode (describe
needs no receptor/prompt); added workflow_describe_api.json. Docs updated with the
first-pass bootstrap step. Fixed error-return arity to 5-tuple.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

workflow/workflow_describe_api.json

@@ -0,0 +1,26 @@
+{
+  "11": {
+    "class_type": "LoadImage",
+    "inputs": { "image": "reference.png" },
+    "_meta": { "title": "Reference Image (put in ComfyUI/input/)" }
+  },
+  "12": {
+    "class_type": "QwenVLImageJudge",
+    "inputs": {
+      "reference_image": ["11", 0],
+      "mode": "describe",
+      "model_path": "/media/p5/qwen3vl_4b_abliterated_comfy_convert/hf_bf16",
+      "precision": "bf16",
+      "axes": "subject_count, gender_mix, body_type, distinctive_features, age_appearance, ethnicity_skin, hair, clothing_state, sexual_act, position, penetration, explicitness, body_contact, pose, facial_expression, gaze, framing, camera_angle, scene, lighting_color, art_style",
+      "max_new_tokens": 1024,
+      "temperature": 0.0,
+      "swap_eval": false,
+      "keep_loaded": true,
+      "auto_download": true,
+      "report_dir": "/media/p5/Comfyui/output/calibrator",
+      "run_tag": "seed",
+      "prompt_used": ""
+    },
+    "_meta": { "title": "Qwen3-VL Describe (first pass)" }
+  }
+}