chore(meta): document modifier coin base in asset-gen skill

Adds the coin base PNG location and the prompt pattern that
keeps the model anchored to the correct coin style.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/skills/asset-gen/SKILL.md

@@ -79,6 +79,26 @@ When generating multiple assets of the same type:
 5. Save PNGs to correct paths after each batch.
 6. Report summary: count generated, failures, paths written.
 
+## Modifier Coin Base
+
+All modifier icons use a shared hammered-metal coin base for visual cohesion.
+Pass it as `inputImagePath` when generating modifier icons:
+
+```
+~/Pictures/mcp-images/icons/modifiers/modifier_base_coin.png
+```
+
+Use the stronger prompt pattern to keep the model on the coin base:
+
+> On a weathered dark steel-silver coin with rough beveled edges and warm
+> gold-amber rim glow, against a dark navy-charcoal swirled background.
+> CK3 game modifier icon, painterly style. Embossed relief on the coin
+> surface of [MOTIF]. The motif is engraved into the smooth grey coin face
+> as a bas-relief.
+
+If the model drifts (different edge style, wrong background), regenerate — the
+coin base consistency is critical for the set to look cohesive at 60x60.
+
 ## DDS Conversion
 
 CK3 requires DDS format. After generating PNGs, convert using the format