Add --git option for explicit git mv control

[Copilot]

Issue #84 requested evaluation of git mv benefits and adding it as an option. GitStorageManager (using LibGit2Sharp's Commands.Move) already existed but was only available via auto-detection. This adds explicit control via --git flag.

Changes

• New --git flag on update command: Forces use of GitStorageManager, errors if not in a git repository
• New constructor: ListingManager(DirectoryInfo, bool useGit) for explicit storage manager selection
• Fixed git detection: Changed from Repository.IsValid() to Repository.Discover() to properly detect repos from subdirectories
• Maintained backward compatibility: Default constructor auto-detects, preferring GitStorageManager when available

Behavior

# Force git mv (errors if not in repo)
ListingManager update Chapter03 --git

# Auto-detect (existing behavior, now works from subdirs)
ListingManager update Chapter03

Scenario	Result
--git in git repo	Uses GitStorageManager
--git outside git repo	Error with clear message
No --git in git repo	Auto-detects, uses GitStorageManager
No --git outside git repo	Auto-detects, uses OSStorageManager

Why git mv matters

Preserves file rename history for git log --follow, blame, and code review.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Move files using git mv</issue_title>
<issue_description>Evaluate if git mv provides a significant difference in the way moved files are treated by git, and consider adding it as an option. I.E: --git

This was started in #84 . There may not be, but see if enhancements can be made</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Move files using git mv #4

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