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...

docs/getting-started.md

@@ -255,3 +255,6 @@ See the [Skills & Agents Factory](https://github.com/alirezarezvani/claude-code-
 
 ??? question "Does this work with Cursor, Windsurf, Aider, or other tools?"
     Yes. All 156 skills can be converted to native formats for Cursor, Aider, Kilo Code, Windsurf, OpenCode, Augment, and Antigravity. Run `./scripts/convert.sh --tool all` and then install with `./scripts/install.sh --tool <name>`. See [Multi-Tool Integrations](integrations.md) for details.
+
+??? question "Can I use Agent Skills in ChatGPT?"
+    Yes. We have [6 Custom GPTs](custom-gpts.md) that bring Agent Skills directly into ChatGPT — no installation needed. Just click and start chatting.