feat: comprehensive registry improvements

Community docs:
- CODE_OF_CONDUCT.md (Contributor Covenant v2.1)
- SECURITY.md (vulnerability reporting policy)
- CHANGELOG.md (initial release notes)
- CODEOWNERS (per-type review ownership)

GitHub config:
- Issue templates: bug-report, pricing-correction, documentation
- FUNDING.yml (GitHub Sponsors)

Validation enhancements:
- Cross-reference check: hand [[requires]] → integration existence
- Routing alias collisions as warnings (errors with --strict)
- --strict flag to promote warnings to errors
- --type filter to validate single content type
- CI: add taplo format check and lychee link check jobs

Developer experience:
- Makefile with validate, fmt, and scaffold targets
- Scaffold templates for all 5 content types
- .pre-commit-config.yaml (trailing whitespace, TOML check, validate)
- docs/content-guide.md (naming, descriptions, prompts, decision guide)

Content quality:
- schema.toml: add last_verified field for model pricing
- CONTRIBUTING.md: add pricing verification guide with source links

CODE_OF_CONDUCT.md

@@ -0,0 +1,29 @@
+# Code of Conduct
+
+## Our Pledge
+
+We pledge to make participation in our community a harassment-free experience for everyone, regardless of age, body size, disability, ethnicity, gender identity and expression, level of experience, nationality, personal appearance, race, religion, or sexual identity and orientation.
+
+## Our Standards
+
+Examples of behavior that contributes to a positive environment:
+
+- Using welcoming and inclusive language
+- Being respectful of differing viewpoints and experiences
+- Gracefully accepting constructive criticism
+- Focusing on what is best for the community
+
+Examples of unacceptable behavior:
+
+- Trolling, insulting/derogatory comments, and personal or political attacks
+- Public or private harassment
+- Publishing others' private information without explicit permission
+- Other conduct which could reasonably be considered inappropriate
+
+## Enforcement
+
+Instances of unacceptable behavior may be reported to **team@librefang.dev**. All complaints will be reviewed and investigated promptly and fairly.
+
+## Attribution
+
+This Code of Conduct is adapted from the [Contributor Covenant](https://www.contributor-covenant.org/), version 2.1.