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

schema.toml

@@ -27,6 +27,7 @@ context_window = 128000                # Required: maximum input tokens
 max_output_tokens = 16384              # Required: maximum output tokens
 input_cost_per_m = 2.50                # Required: USD per million input tokens (0.0 for free/local)
 output_cost_per_m = 10.0               # Required: USD per million output tokens (0.0 for free/local)
+last_verified = "2025-03-15"           # Optional: ISO date when pricing was last verified against official sources
 supports_tools = true                  # Optional: tool/function calling support (default: false)
 supports_vision = true                 # Optional: vision/image input support (default: false)
 supports_streaming = true              # Optional: streaming response support (default: true)