Add GitHub Repository Showcase with resilient data fallback

[markhazleton]

Summary

Implements the GitHub Repository Showcase feature for BootstrapSpark with a dedicated route, repository data service, proxy integration, fallback handling, and phased spec/task updates. The implementation also separates source files from build artifacts so repeated builds do not modify tracked source data.

Changes

• Added repository domain model, feed validation, and view-model mapping.
• Added repository data service with remote, cache, build-artifact, and embedded fallback behavior.
• Added repository page route and Apps navigation entry.
• Added sitemap route generation for the repositories page.
• Added repository proxy endpoint for production and dev proxy support.
• Added repository feed sync script and integrated it into build.
• Redirected build-time repository sync output to ignored build artifacts.
• Added contract, unit, and integration tests for repository feature surfaces.
• Updated feature spec/tasks status and gate acknowledgement records.

Task Completion

• 19/31 tasks complete.
• Checklist status: 26/26 complete.

Quality Gates

• Required gates: checklist, analyze, critic.
• Current artifacts found: checklist only.
• No analyze or critic gate artifact currently present.

Gate Acknowledgements

• 2026-04-17: user approved proceeding despite unresolved analyze and critic gate artifacts.

Spec Reference

• .documentation/specs/001-github-repo-showcase

Quickfix Reference

• N/A

Notes

• There are local uncommitted changes in the branch.
• This PR is intentionally opened as draft until remaining tasks and gates are finished.