feat: JSON Schema validation for metadata.yaml as pre-commit hook

Adds schema/metadata.json (JSON Schema draft-07) as the authoritative
schema for all rpms/*/metadata.yaml files.  Key constraints:

- additionalProperties: false at both the top level and builds items,
  so unknown fields are rejected outright
- name allows dots (fixes neovim-glibc-2.17)
- github is optional (fixes claude-code, which has no GitHub repo)
- repository enum includes fedora/42, fedora/43, fedora/44
- dist_tag boolean field documented in schema

Pre-commit hook (check-jsonschema 0.37.2) runs against every file
matching ^rpms/[^/]+/metadata\.yaml$ on every commit.

Also fixes rpms/claude-code/metadata.yaml: removes the unknown
claude_ai field and normalises field order.

.pre-commit-config.yaml

@@ -31,6 +31,14 @@ repos:
             "-s",
           ]
 
+  - repo: https://github.com/python-jsonschema/check-jsonschema
+    rev: 0.37.2
+    hooks:
+      - id: check-jsonschema
+        name: Validate RPM package metadata
+        files: ^rpms/[^/]+/metadata\.yaml$
+        args: [--schemafile, schema/metadata.json]
+
   - repo: https://github.com/astral-sh/ruff-pre-commit
     rev: v0.14.7
     hooks: