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

templates/skill.toml

@@ -0,0 +1,20 @@
+[skill]
+name = "{{NAME}}"
+version = "0.1.0"
+description = "TODO: One-sentence description of what this skill does."
+author = "TODO"
+tags = ["TODO"]
+
+[runtime]
+type = "promptonly"
+
+[input]
+topic = { type = "string", description = "TODO: Describe this input parameter", required = true }
+
+[prompt]
+template = """TODO: Write the prompt template for this skill.
+
+Use {{topic}} to reference input parameters.
+
+Be specific about the desired output format and constraints.
+"""