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

.github/ISSUE_TEMPLATE/documentation.yml

@@ -0,0 +1,23 @@
+name: Documentation Issue
+description: Report incorrect, incomplete, or missing documentation
+labels: ["documentation"]
+body:
+  - type: input
+    id: which_document
+    attributes:
+      label: Which Document
+      description: The file path or page where the issue exists.
+      placeholder: e.g. README.md, docs/contributing.md, providers/openai.toml
+    validations:
+      required: true
+
+  - type: textarea
+    id: whats_wrong
+    attributes:
+      label: What's Wrong or Missing
+      description: Describe the documentation issue.
+      placeholder: |
+        e.g. The contributing guide doesn't explain how to add a new provider,
+        or the README setup instructions are outdated.
+    validations:
+      required: true