refactor(vcs): back GitRepository with LibGit2Sharp

[ChrisonSimtian]

What

• Replace the hand-rolled .git/config, HEAD and packed-refs parsing in GitRepository.FromLocalDirectory with LibGit2Sharp introspection (head, commit, branch, tags, tracked remote/branch, remote URL).
• Replace the remote-URL regex with Uri-based parsing, plus a small SCP-like (git@host:path) fallback for the syntax Uri cannot represent.
• Add LibGit2Sharp 0.31.0 to Directory.Packages.props and reference it from Fallout.Build.

Why

• The old code parsed git on-disk internals by hand. It breaks whenever git changes its storage format. LibGit2Sharp is the standard binding and removes the fragile parsing + URL regex.
• Public GitRepository surface is unchanged. This is an internal swap, non-breaking, targeting the 2026 line.

Tests

• GitRepositoryTest green: 26 passed (was 22), added SSH-with-port, HTTP, file:// and UNC (\server\share\repo.git) remote cases.
• dotnet build fallout.slnx succeeds, 0 errors.

Note on base branch

The task asked to base this on spike/shim-migration-redesign. This worktree was actually forked from main (b4866d92), which still uses the src/Fallout.Build/ layout, whereas spike/shim-migration-redesign has already renamed the project to src/Fallout.Application/ (onion re-layout). Basing against the spike branch would produce a huge spurious diff and land the edit on a path that no longer exists there, so this PR targets main where the change applies cleanly. Re-target if the spike branch is the intended integration point.

Closes #352

[ChrisonSimtian]

Discarding. The agent worktrees forked from the pre-rearchitecture tree (main), so this is implemented against old paths (Fallout.Build/Common/Utilities) and does not stack cleanly on #359. Will redo on the new onion structure after #359 merges. Issue stays open.