feat: dist_tag boolean for distro-aware release strings

Adds a per-package dist_tag: true/false metadata flag (default false).
When enabled the build tool appends the RPM dist tag to the release
at build time so each distro produces a unique version in the Gitea
package registry:

  release: 1  +  almalinux/el9  ->  PACKAGE_RELEASE=1.el9
  release: 1  +  fedora/43      ->  PACKAGE_RELEASE=1.fc43

This ensures Gitea package existence checks don't confuse packages built
for one distro with those built for another (the original bug).

Changes:
- Add effective_release() and get_rpm_dist_tag() helpers
- Revert the broken files-endpoint check from the previous commit;
  dist disambiguation is now handled purely via the release string
- discover_packages and build_single both compute the effective release
  before constructing PackageInfo
- check_package_exists simplified back to a single version-level check
- dist_tag added to METADATA_SCHEMA and PackageMetadata dataclass
- All 72 metadata.yaml files updated with dist_tag: true

rpms/claude-code/metadata.yaml

@@ -1,4 +1,11 @@
+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
+dist_tag: true
 builds:
 - image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
   release: 1
@@ -11,9 +18,3 @@ builds:
   - almalinux/el9
   version: 2.1.126
 claude_ai: true
-description: Claude Code - Anthropic's agentic AI coding tool
-homepage: https://claude.ai/code
-license: Proprietary
-maintainer: Anthropic
-name: claude-code
-platform: linux