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.

rpms/claude-code/metadata.yaml

@@ -1,19 +1,18 @@
-arch: amd64
-builds:
-- image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
-  release: 1
-  repository:
-  - almalinux/el8
-  version: 2.1.126
-- image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
-  release: 1
-  repository:
-  - almalinux/el9
-  version: 2.1.126
-claude_ai: true
+name: claude-code
 description: Claude Code - Anthropic's agentic AI coding tool
+arch: amd64
+platform: linux
+maintainer: Anthropic
 homepage: https://claude.ai/code
 license: Proprietary
-maintainer: Anthropic
-name: claude-code
-platform: linux
+builds:
+- repository:
+  - almalinux/el8
+  image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
+  release: 1
+  version: 2.1.126
+- repository:
+  - almalinux/el9
+  image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
+  release: 1
+  version: 2.1.126