Initial release: diagram-design v1.0

Thirteen editorial diagram types (architecture, flowchart, sequence,
state, ER, timeline, swimlane, quadrant, nested, tree, layers, venn,
pyramid) as self-contained HTML files with inline SVG.

Progressive disclosure: lean SKILL.md points to per-type references.
Neutral stone+rust default skin, first-run gate that prompts users
to customize the style guide from their own website, URL-based
onboarding, annotation-callout primitive, sketchy filter variant,
live gallery at assets/index.html.

references/type-er.md

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+# ER / Data Model
+
+**Best for:** database schemas, API resource relationships, domain models.
+
+## Layout conventions
+- Each entity is a two-section box:
+  - **Header**: type tag (`ENTITY`) + entity name in Geist.
+  - **Body**: field list in Geist Mono, one per line. PK prefixed with `#`, FK prefixed with `→`.
+- Relationships: lines between entities with cardinality at each end:
+  - `1`, `N`, `0..1`, `1..*` in Geist Mono, 8px, placed 10–12px from the entity edge.
+  - Optional relationship label ("has", "belongs to") centered on the line.
+- Group related entities close; lay out so most relationships are straight lines, not tangles.
+- Coral on the aggregate root or central entity of the model.
+
+## Anti-patterns
+- Drawing an arrow for every FK on a model with dozens — lay out by cluster instead.
+- Inconsistent cardinality notation between ends of the same relationship.
+- Fields padded to equal-height boxes — natural height by content is fine.
+
+## Examples
+- `assets/example-er.html` — minimal light
+- `assets/example-er-dark.html` — minimal dark
+- `assets/example-er-full.html` — full editorial