docs: add AI Traceability & Attribution guide

Comprehensive documentation on AI code attribution and disclosure:

- New guide: guide/ai-traceability.md (~640 lines)
  - LLVM "Human-in-the-Loop" policy (Assisted-by trailer)
  - Ghostty mandatory disclosure pattern
  - Fedora contributor accountability framework
  - git-ai tool documentation
  - PromptPwnd security vulnerability
  - Four-level disclosure spectrum
  - Implementation guides (solo, team, enterprise)

- Templates: examples/config/
  - CONTRIBUTING-ai-disclosure.md
  - PULL_REQUEST_TEMPLATE-ai.md

- Cross-references added to:
  - ultimate-guide.md (after Co-Authored-By section)
  - learning-with-ai.md (after Vibe Coding Trap)
  - security-hardening.md (See Also)
  - guide/README.md (table of contents)

- reference.yaml: 14 new entries for AI traceability topics

Source: Vibe coding needs git blame (Piotr Migdał, Jan 2026)
+ Perplexity research on LLVM, Ghostty, Fedora policies

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

guide/ultimate-guide.md

@@ -9253,6 +9253,15 @@ git commit -m "feat: add user authentication middleware
 Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>"
 ```
 
+#### AI Attribution Standards
+
+For comprehensive AI code attribution beyond Co-Authored-By, including:
+- LLVM's `Assisted-by:` trailer standard
+- git-ai checkpoint tracking
+- Team and enterprise compliance patterns
+
+See: [AI Traceability Guide](./ai-traceability.md)
+
 **Customize commit style in CLAUDE.md:**
 
 ```markdown