mAi: #10 - multi-model backend expansion (workflow templates + compare harness)

Path 1 architecture: one comfyui adapter, workflows as data.

- workflow_template.go: embed.FS + token substitution with type-preserving
  whole-value placeholders. ${prompt} → string, ${seed} → int64,
  ${cfg} → float64 — no JSON round-tripping. Partial matches ignored.
- comfyui.go: refactored to load workflow from embedded FS or filesystem
  path. Back-compat preserved: workflow: defaults to flux1-schnell.
- workflows/{flux1-schnell,flux2-klein,sd35-medium}.json — bundled
  templates. flux1-schnell migrated from hardcoded with identical node IDs.
- compare.go: new `imagen compare` subcommand. Sequential N-backend run
  (one GPU on mRock — parallel would OOM), per-backend PNG, sidecar JSON
  with per-model metadata + errors, composite contact sheet via Go image
  package (no ImageMagick dep).
- Sample config gains flux2-klein-local + sd35-medium-local instances.
- docs/backends.md: architecture rationale + per-model HF download paths
  + how to add a new bundled workflow + compare-harness reference.

Live smoke verified: compare mock + flux-schnell-local at 768×768 →
both PNGs written, sidecar JSON has workflow="flux1-schnell" + full
metadata, contact sheet renders. Worker contract (Request → Generate)
unchanged, so flexsiebels /imagine UI API surface preserved.

Tests: 11 existing comfyui + 6 new workflow_template + 5 new compare
tests, all green.

Adding a new model is now yaml + JSON, never Go.

docs/usage.md

@@ -4,14 +4,21 @@
 
 ```
 imagen generate <prompt> [flags]    generate one image
+imagen compare <prompt> --models a,b,c [flags]
+                                    run one prompt across N backends + contact sheet
+imagen worker [flags]               consume the imagen.jobs queue (daemon)
 imagen backends                     list configured + registered backends
 imagen config init                  print a sample imagen.yaml on stdout
 imagen config validate              parse + validate the active config
 imagen config path                  print the resolved config path
 imagen serve [--addr :8080]         (stub) start the HTTP server
+imagen usage [--since DATE]         show cost-tracking rows
 imagen version                      print version
 ```
 
+For the per-backend setup (FLUX.1, FLUX.2 [klein], SD3.5 medium, …) and
+the architecture rationale, see [`backends.md`](backends.md).
+
 ## `generate` flags
 
 | Flag           | Default                      | Notes                                                       |