feat: add image skill for AI image generation and marketing visuals (#259)

* feat: add image skill for AI image generation and marketing visuals

Covers AI image generation (Gemini, Flux, Ideogram, DALL-E, Midjourney),
design tools (Canva, Figma), marketing workflows (blog heroes, social
graphics, product mockups, profile/listing banners, brand assets), image
optimization, and OG/social preview images.

Includes prompting reference guide with model-specific tips.

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

* fix: apply accuracy fixes from Codex review

- OpenAI: DALL-E 3 → GPT Image family (DALL-E 3 deprecated)
- Gemini: use official naming, keep Nano Banana as context
- Ideogram: remove ungrounded ~90% accuracy claim
- All pricing: replace hard numbers with doc links
- LinkedIn company cover: note recommended 4200x700
- WebP/AVIF: adjust browser support to ~96%/~94%
- Flux: remove specific reference image count (varies by variant)

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -12,7 +12,7 @@
   "plugins": [
     {
       "name": "marketing-skills",
-      "description": "39 marketing skills for technical marketers and founders: ASO, CRO, copywriting, cold email, SEO, AI SEO, paid ads, ad creative, video production, churn prevention, pricing strategy, referral programs, revenue operations, sales enablement, customer research, site architecture, and more",
+      "description": "40 marketing skills for technical marketers and founders: ASO, CRO, copywriting, cold email, SEO, AI SEO, paid ads, ad creative, video production, image generation, churn prevention, pricing strategy, referral programs, revenue operations, sales enablement, customer research, site architecture, and more",
       "source": "./"
     }
   ]