feat(baoyu-imagine): add DashScope Wan 2.7 image model support (#141)

* feat(baoyu-imagine): add DashScope Wan 2.7 image model support

Closes #139.

Adds the new `wan2.7-image-pro` and `wan2.7-image` model family to the
DashScope provider so users can call Wan 2.7 directly through the
official Aliyun (Bailian) API instead of going through Replicate.

- Register `wan2.7-image-pro` and `wan2.7-image` as a new `wan27` family
  in the DashScope provider with their own size resolution rules:
  pixel range `[768*768, 4096*4096]` for `wan2.7-image-pro` text-to-image,
  `[768*768, 2048*2048]` for `wan2.7-image-pro` with refs and for the
  base `wan2.7-image` model in any mode, with aspect ratios validated
  against the documented `[1:8, 8:1]` band.
- Allow up to 9 reference images per request (image editing /
  multi-image fusion). Local files are inlined as base64 data URLs;
  `http(s)://` paths are forwarded as-is. Other DashScope models still
  reject `--ref` with a hint to switch to a wan2.7 model or another
  provider.
- Drop `prompt_extend` from the request body for the Wan 2.7 family
  (not part of the Wan 2.7 API surface) and skip the Qwen-only negative
  prompt for this family.
- Allow `--provider dashscope --ref ...` in `detectProvider` so users
  can opt into Wan 2.7 reference workflows, while keeping Wan 2.7 out
  of the auto-detect ref priority list.
- Add provider, reference, and usage-example documentation, plus
  unit tests covering family routing, size derivation across the
  three pixel-budget modes, ratio rejection, explicit-size validation,
  and the new `--provider dashscope` ref opt-in path.

Made-with: Cursor

* fix(baoyu-imagine): force n=1 for DashScope wan2.7 to avoid silent multi-image billing

Cross-checked the implementation against the official Wan 2.7 image
generation & editing API reference and found that the API defaults
`parameters.n` to 4 in non-collage mode (1-4 range, billed per image).
baoyu-imagine has single-image save semantics — only the first image
in the response is kept — so without an explicit `n: 1` users would
silently pay for 3 discarded images per request.

- Always send `parameters.n: 1` in the wan2.7 request body
- Reject `--n > 1` for wan2.7 with a clear error pointing at the
  single-image save semantics
- Add tests asserting the request body shape (n=1, no prompt_extend,
  no negative_prompt) and the --n>1 rejection
- Document the defaults-vs-skill mismatch in the dashscope reference

Made-with: Cursor

* Fix DashScope Wan 2.7 review feedback

skills/baoyu-imagine/references/usage-examples.md

@@ -51,6 +51,12 @@ ${BUN_X} {baseDir}/scripts/main.ts --prompt "为咖啡品牌设计一张 21:9
 # DashScope legacy fixed-size
 ${BUN_X} {baseDir}/scripts/main.ts --prompt "一张电影感海报" --image out.png --provider dashscope --model qwen-image-max --size 1664x928
 
+# DashScope Wan 2.7 Image Pro (4K text-to-image)
+${BUN_X} {baseDir}/scripts/main.ts --prompt "一间有着精致窗户的花店" --image out.png --provider dashscope --model wan2.7-image-pro --size 4096x4096
+
+# DashScope Wan 2.7 Image with reference image (multi-image fusion)
+${BUN_X} {baseDir}/scripts/main.ts --prompt "把图2的涂鸦喷绘在图1的汽车上" --image out.png --provider dashscope --model wan2.7-image-pro --ref car.webp paint.webp
+
 # Z.AI GLM-image
 ${BUN_X} {baseDir}/scripts/main.ts --prompt "一张带清晰中文标题的科技海报" --image out.png --provider zai