Add machine-readable registry.yaml, generate README table

[fzipi]

Summary

• Adds registry.yaml as the authoritative, machine-readable plugin index (rule ID range, repository, type, status, license, CI, private), validated against registry-schema.json in CI.
• README.md's table and registry.json are generated from registry.yaml by scripts/generate_registry.py. CI regenerates and fails on drift (git diff --exit-code) instead of committing from the workflow.
• Migrating the table fixes the drift found in the audit: swapped CI badges between nextcloud/dokuwiki (now derived from each plugin's own repository field, making the swap structurally impossible), the missing License cell on false-positive-report-plugin, dokuwiki's link-text typo, missing CI badges on body-decompress-plugin and traffic-observation-plugin despite having integration tests, and documents the undocumented 9,515,000-9,515,999 hole as a reserved range.
• Also corrects vimbadmin-crs-plugin's license, which the table listed as GPL 2.0 but is actually Apache-2.0 per the repository's detected license.
• Adds docs/registry-schema.md documenting the fields and, per the issue, what the registry attests and what it does not (not a code audit or supply-chain guarantee).
• Adds .github/workflows/validate-registry.yml: schema validation via check-jsonschema, semantic validation (unique names, no overlapping rule ID ranges — constraints JSON Schema can't express) via a small script-based test, and the drift check.

Refs #21.

Test plan

• uvx check-jsonschema --schemafile registry-schema.json --force-filetype yaml registry.yaml tests/registry/valid/*.yaml passes
• Each fixture under tests/registry/invalid/*.yaml is rejected by the schema
• uv run tests/registry/test_validate.py passes (duplicate-name and overlapping-range fixtures under tests/registry/invalid-semantic/ are correctly rejected)
• uv run scripts/generate_registry.py regenerates README.md and registry.json with no drift against the committed versions
• Existing plugin descriptor validation (plugin-schema.json, examples, plugin.yaml.template) still passes unaffected

🤖 Generated with Claude Code