Migrate Get-Branch: list branches via for-each-ref with typed output

[shmuelie]

Part of #52. Depends on the shared git-invocation helper sub-issue.

Goal

Add a Get-Branch command to Shmuelie.Git that lists local and remote
branches with tracking info, emitting a typed object — modernizing
ObjectiveGit's Get-Branch.

Modernize (do not port as-is)

• ObjectiveGit scrapes git branch --no-color -vv --no-abbrev (and -r) with a
  stack of fragile regexes. Replace with a machine-readable format:
  git for-each-ref --format=... (or --porcelain-style), consistent with the
  hardening in Harden git output parsing: porcelain rename/quotePath (tab completion) and locale-dependent fetch classification #33 (handle quotePath/rename/unusual ref names).
• Return a typed object (like Get-Worktrees / Get-GitStatusSummary), not an
  ad-hoc PSCustomObject shape.
• Cross-platform repository path handling (no '.\' string default; use
  Join-Path/[System.IO.Path]).

Suggested shape

Per-branch: name, whether current, commit hash, upstream/tracks, ahead/behind,
symbolic target (for HEAD -> ...), local vs remote, last commit subject.

Acceptance criteria

• Exported from both the .psm1 Export-ModuleMember list and the .psd1
FunctionsToExport list.
• Comment-based help.
• Uses the shared git-invocation helper; no Invoke-Expression.
• Works on non-Windows PowerShell 7.
• README command table + modules/Shmuelie.Git/CHANGELOG.md [Unreleased]
  updated.
• Deterministic Pester tests (synthetic branch data / $TestDrive temp repo
  only where needed).
• build/Test-Modules.ps1 and build/Invoke-Tests.ps1 pass.