feat: Custom GPTs page, gitignore configs, SEO README, cross-references

- Gitignore individual GPT config files (custom-gpt/*-gpt.md) — instructions
  should not be publicly exposed
- Rewrite custom-gpt/README.md with SEO-optimized descriptions and live
  ChatGPT links for all 6 GPTs
- New docs page: docs/custom-gpts.md with grid cards, comparison table,
  and stats
- Add Custom GPTs to mkdocs.yml navigation
- Cross-reference Custom GPTs from:
  - Homepage (new grid card)
  - Getting Started (FAQ entry)
  - Personas page (try in ChatGPT callout)
  - Plugins page (FAQ entry)

Live GPT links:
- Solo Founder: chatgpt.com/g/g-69b315...
- SEO Audit Expert: chatgpt.com/g/g-69b3b0...
- Content Strategist: chatgpt.com/g/g-69b3af...
- Product Manager Toolkit: chatgpt.com/g/g-69b32c...
- Conversion Copywriter: chatgpt.com/g/g-69b327...
- CTO Advisor: chatgpt.com/g/g-69b326...

mkdocs.yml

@@ -324,6 +324,7 @@ nav:
     - "Growth Marketer": personas/growth-marketer.md
     - "Solo Founder": personas/solo-founder.md
   - Orchestration: orchestration.md
+  - Custom GPTs: custom-gpts.md
   - Agents:
     - Overview: agents/index.md
     - "CS Agile Product Owner": agents/cs-agile-product-owner.md