feat: add content targets and loop expansion for target templates

Content targets write literal text to files via 'content:' field,
without requiring an AI provider or API keys. They are not archived
when overwritten.

Loop expansion allows defining 'loops:' at the top level with named
lists of values. Targets with [var] in their name are expanded via
cartesian product. Variables are substituted in all string fields.
Explicit targets override expanded ones. Escaping: \[var] -> [var].
Expansion happens at config load time so the rest of the system
(builder, graph, state) sees only expanded targets.

README.md

@@ -67,6 +67,7 @@ The config file must be named `<anything>.hokusai.yaml` and placed in your proje
 | Field | Description |
 |---|---|
 | `defaults` | Default model names (optional) |
+| `loops` | Loop variables for target template expansion (optional) |
 | `archive_folder` | Directory to move previous outputs into before rebuilding (optional) |
 | `targets` | Map of output filenames to their configuration |
 
@@ -90,6 +91,7 @@ defaults:
 | `width` | int | Image width in pixels |
 | `height` | int | Image height in pixels |
 | `download` | string | URL to download instead of generating (mutually exclusive with prompt) |
+| `content` | string | Literal text to write to the file (mutually exclusive with prompt/download) |
 
 Target type is inferred from the file extension:
 - **Image**: `.png`, `.jpg`, `.jpeg`, `.webp`
@@ -151,6 +153,94 @@ targets:
 
 Download targets participate in dependency resolution like any other target. They are skipped if the URL hasn't changed.
 
+### Content targets
+
+Targets can write literal text content directly to a file without invoking any AI provider:
+
+```yaml
+targets:
+  config.txt:
+    content: "Some static configuration"
+
+  data.csv:
+    content: |
+      name,value
+      alpha,1
+      beta,2
+```
+
+Content targets don't require API keys and are not archived when overwritten. They participate in dependency resolution like any other target, so generated targets can depend on them.
+
+### Loops
+
+Define `loops` at the top level to generate multiple targets from a template using cartesian products:
+
+```yaml
+loops:
+  color:
+    - red
+    - blue
+    - green
+  size:
+    - small
+    - large
+
+targets:
+  card-[color]-[size].png:
+    prompt: "A [color] card in [size] format"
+    width: 1024
+    height: 768
+```
+
+This expands to 6 targets: `card-red-small.png`, `card-red-large.png`, `card-blue-small.png`, etc. Loop variables are substituted in all string fields: prompts, inputs, reference images, control images, download URLs, and content.
+
+Only variables that appear in the target name cause expansion. A target without any `[var]` references in its name is not looped:
+
+```yaml
+loops:
+  id:
+    - 1
+    - 2
+
+targets:
+  data-[id].txt:
+    content: "Data for [id]"
+
+  # This target depends on ALL expanded data files
+  summary.md:
+    prompt: "Summarize everything"
+    inputs:
+      - data-1.txt
+      - data-2.txt
+```
+
+Loop variables also work across dependent targets:
+
+```yaml
+targets:
+  data-[id].txt:
+    content: "Data for item [id]"
+
+  report-[id].md:
+    prompt: "Write a report about item [id]"
+    inputs:
+      - data-[id].txt
+```
+
+**Explicit overrides**: If you define both a template and an explicit target that would collide, the explicit target wins:
+
+```yaml
+targets:
+  image-[n].png:
+    prompt: "Generic image [n]"
+  image-3.png:
+    prompt: "Special custom image"   # this overrides the template for n=3
+```
+
+**Escaping**: Use `\[var]` to produce a literal `[var]` in the output.
+
+Loop values are always treated as strings. Numbers and booleans in YAML are automatically converted.
+
 ### 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: