docs: update README and CLAUDE.md with recent features

- Add regenerate command documentation
- Add download target type
- Fix reference_images field (list, not single string)
- Document archive behavior for clean command
- Update module structure to reflect actual files
- Add OpenAI provider documentation
- Update supported models list
- Add OPENAI_API_KEY to environment variables

README.md

@@ -85,10 +85,11 @@ defaults:
 | `prompt` | string | Inline prompt text, or path to a prompt file |
 | `model` | string | Override the default model for this target |
 | `inputs` | list[string] | Files this target depends on (other targets or existing files) |
-| `reference_image` | string | Image file for image-to-image generation |
+| `reference_images` | list[string] | Image files for image-to-image generation |
 | `control_images` | list[string] | Control images (for canny/depth models) |
 | `width` | int | Image width in pixels |
 | `height` | int | Image height in pixels |
+| `download` | string | URL to download instead of generating (mutually exclusive with prompt) |
 
 Target type is inferred from the file extension:
 - **Image**: `.png`, `.jpg`, `.jpeg`, `.webp`
@@ -133,6 +134,23 @@ targets:
 
 hokusai resolves dependencies automatically. If you build a single target, its transitive dependencies are included.
 
+### Download targets
+
+Targets can download files from URLs instead of generating them:
+
+```yaml
+targets:
+  reference.jpg:
+    download: https://example.com/image.jpg
+
+  variation.png:
+    prompt: "A variation of this image in watercolor style"
+    reference_images:
+      - reference.jpg
+```
+
+Download targets participate in dependency resolution like any other target. They are skipped if the URL hasn't changed.
+
 ### Archiving previous outputs
 
 Set `archive_folder` at the top level to preserve previous versions of generated files. When a target is rebuilt, the existing output is moved to the archive folder with an incrementing numeric suffix:
@@ -157,10 +175,23 @@ Build all targets, or a specific target and its dependencies.
 - Runs independent targets in parallel
 - Continues building if a target fails (dependents of the failed target are skipped)
 
+### `hokusai regenerate <targets...>`
+
+Force regeneration of specific targets, ignoring their up-to-date status. Useful for getting a new variation of an AI-generated output without changing the prompt.
+
+```bash
+hokusai regenerate hero.png           # regenerate one target
+hokusai regenerate hero.png logo.png  # regenerate multiple targets
+```
+
+If `archive_folder` is set, the previous versions are archived before regeneration.
+
 ### `hokusai clean`
 
 Remove all generated target files and the build state file (`.hokusai.state.yaml`). Input files are preserved.
 
+If `archive_folder` is set, files are moved to the archive instead of being deleted.
+
 ### `hokusai graph`
 
 Print the dependency graph showing build stages: