Migrate Restore-Changes: git stash pop with ShouldProcess

[shmuelie]

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

Goal

Add a command to restore stashed changes (git stash pop) — ObjectiveGit's
Restore-Changes — modernized.

Modernize (do not port as-is)

• ObjectiveGit runs git ... stash pop via Invoke-Expression. Use the shared
  helper with a git argument array.
• Add [CmdletBinding(SupportsShouldProcess)]; honor -WhatIf/-Confirm (pop
  changes working-tree/index state and can conflict).
• Cross-platform repository path handling; drop the magic exit-code / bug branch.
• Consider a parameter to target a specific stash entry (optional enhancement
  beyond the original, which always popped the top).

Acceptance criteria

• Exported from both the .psm1 and .psd1 export lists.
• Comment-based help; SupportsShouldProcess honored.
• Final command name uses an approved verb and fits the module surface (pairs
  naturally with the Backup-Changes replacement).
• README command table + [Unreleased] CHANGELOG updated.
• Deterministic Pester tests.
• build/Test-Modules.ps1 and build/Invoke-Tests.ps1 pass.