mAi: #2 - phase 2 ComfyUI Go adapter, tests, config sample

internal/backend/comfyui.go implements the Backend interface against
ComfyUI's /prompt + /history + /view HTTP API. Workflow is the canonical
FLUX.1 schnell shape — UNETLoader + DualCLIPLoader (clip_l + t5xxl fp8) +
VAELoader + ModelSamplingFlux + KSampler — assembled as a Go map per
request so Width / Height / Seed / Steps / sampler / scheduler all flow
into the right node inputs.

Resilience: one retry on /prompt 5xx and transient network errors, no
retry on 4xx. Connection-refused / timeouts surface a 'boot-whitetower
mrock' hint. node_errors mentioning a missing unet point users at
docs/setup-comfyui-mrock.md (matches both the 4xx and 200-with-errors
shapes ComfyUI uses across versions).

Result.Metadata carries model, seed_used, latency_ms, steps, sampler,
scheduler, width, height, prompt_id, client_id, plus best-effort
vram_used_mib pulled from /system_stats post-gen.

Tests use httptest with poll interval squashed to 1ms — no real mRock
dependency. Coverage: happy path, defaults, retry-once on 5xx, give-up
after two 5xx, no-retry on 4xx, missing-model hint (both 4xx and
200+node_errors paths), history-error surfaced, /view 4xx, unreachable
host, ctx cancel during poll, workflow-shape assertion, registration.

Config sample: flux-schnell-local is now default_backend; the user-facing
block names the unet file by basename (the mapping into models/unet/ is
the server's convention, captured in docs/setup-comfyui-mrock.md from
phase 1).

Smoke verified end-to-end: imagen generate ... --backend
flux-schnell-local --size 1024x1024 --output /tmp/cat-via-cli.png on
mRock returned a 1024x1024 PNG of a cat in a fishbowl in 10.3s with a
sidecar carrying seed + latency_ms + the rest of the metadata.

internal/config/config.go

@@ -95,7 +95,7 @@ const Sample = `# imagen.yaml — config for the imagen CLI.
 # implementing the Backend interface, registering its type name, and listing
 # an instance here.
 
-default_backend: mock
+default_backend: flux-schnell-local
 
 output:
   directory: ~/Pictures/imagen
@@ -103,14 +103,18 @@ output:
   write_metadata_json: true
 
 backends:
-  mock:
-    type: mock
-
   flux-schnell-local:
     type: comfyui
     base_url: http://mrock:8188
+    # Filename of the unet checkpoint inside the ComfyUI server's
+    # models/unet/ directory. See docs/setup-comfyui-mrock.md.
     model: flux1-schnell.safetensors
     default_steps: 4
+    default_sampler: euler
+    default_scheduler: simple
+
+  mock:
+    type: mock
 
   flux-dev-replicate:
     type: replicate