The nbgv install command was hardcoding "master" as the first public release branch in the generated version.json file, regardless of the actual default branch name in the repository. This caused issues in repositories using "main", "develop", or other branch names as their default.

Changes Made

This PR implements automatic default branch detection that works with all GitContext types:

LibGit2Context Implementation

For repositories using LibGit2Sharp (ReadWrite engine), the implementation:

1. Check remotes for HEAD reference - prioritizing upstream > origin > any other remote
2. Fall back to single local branch - if only one branch exists, use it
3. Use git config setting - check init.defaultBranch if the branch exists locally
4. Use common branch names - check for master, main, develop in that order
5. Default to "master" - as final fallback

Alternative Implementation for Other Contexts

For ManagedGitContext (ReadOnly engine) and other context types, the implementation reads .git directory files directly:

• Reads refs/remotes/{remote}/HEAD files for remote HEAD references
• Scans refs/remotes/ directory to find available remotes
• Reads refs/heads/ directory and packed-refs file for local branches
• Parses .git/config and global config for init.defaultBranch setting

Both implementations follow the same priority algorithm to ensure consistent behavior across all git engines.

Example Behavior

Before:

{
  "version": "1.0-beta",
  "publicReleaseRefSpec": [
    "^refs/heads/master$",  // Always master, even in main-based repos
    "^refs/heads/v\\d+(?:\\.\\d+)?$"
  ]
}

After (in a main-based repo):

{
  "version": "1.0-beta", 
  "publicReleaseRefSpec": [
    "^refs/heads/main$",    // Correctly detects main as default
    "^refs/heads/v\\d+(?:\\.\\d+)?$"
  ]
}

Testing

• All existing InstallCommand tests continue to pass
• Manual testing verifies correct behavior with master/main/develop branch scenarios
• Implementation works with both LibGit2Context (ReadWrite) and ManagedGitContext (ReadOnly) engines
• Maintains backward compatibility with existing repositories

Fixes #717.

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