fix: handle missing git repo gracefully in github-metadata plugin

[osteele]

Summary

• The jekyll-github-metadata plugin previously panicked when it couldn't determine the current repository (e.g. when the site source isn't a git repo, or has no GitHub remote)
• Now it logs a warning and continues the build without GitHub metadata, instead of crashing

What changed

plugins/github_metadata.go: Both getCurrentRepo and getGitHubRepo errors are now caught, logged as warnings, and the plugin returns nil to allow the build to proceed.

Repro

Before this fix, building a site that uses github-pages plugin from a non-git directory (e.g. a tarball download) caused a panic:

panic: site drop initialization failed: running plugin: exit status 128

After this fix:

jekyll-github-metadata: can't determine current repository: exit status 128
wrote 151 files in 1.06s.

Test plan

• go test ./... — all tests pass
• Build sd17spring.github.io from tarball (no .git directory) — warns and builds 151 files
• Existing sites with proper git remotes continue to work as before