Add Get-GitHubGitReference and a -Ref tab completer #642

Context: Many functions in this module accept a -Ref/-ref parameter (branch, tag, or SHA) but there is no way to discover valid values or tab-complete them. The Git category (src/functions/public/Git) currently only has Get-GitHubGitConfig and Set-GitHubGitConfig.

Request: Add a Get-GitHubGitReference function wrapping the GitHub REST API get/list reference endpoints:

• Get a reference: https://docs.github.com/en/enterprise-cloud@latest/rest/git/refs?apiVersion=2026-03-10#get-a-reference
• List matching references: https://docs.github.com/en/enterprise-cloud@latest/rest/git/refs?apiVersion=2026-03-10#list-matching-references

Then use it to back a Register-ArgumentCompleter for the -Ref parameter (following the existing pattern in src/functions/public/Workflows/completers.ps1 and src/functions/public/Teams/completers.ps1), so branches/tags can be tab-completed on commands like Start-GitHubWorkflow.

Acceptance criteria:

• Get-GitHubGitReference returns matching refs (branches/tags) for an owner/repository, following existing function conventions in the Git category
• A completers.ps1 is added under src/functions/public/Git (or the relevant categories) registering an ArgumentCompleter for -Ref parameters across applicable functions
• Tab-completing -Ref on a function like Start-GitHubWorkflow suggests branch/tag names
• Follows existing module conventions for pipeline support, output typing, and tests

Related:

• https://docs.github.com/en/enterprise-cloud@latest/rest/git/refs?apiVersion=2026-03-10#get-a-reference
• https://docs.github.com/en/enterprise-cloud@latest/rest/git/refs?apiVersion=2026-03-10#list-matching-references