Migrate Backup-Changes: git stash push (fix --include-untracked & -Message bugs)

[shmuelie]

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

Goal

Add a command to stash changes (git stash push) — ObjectiveGit's
Backup-Changes — with -KeepIndex, -IncludeUntracked, -All, and
-Message, modernized.

Modernize (do not port as-is)

• ObjectiveGit builds the command string and runs it via Invoke-Expression.
  Use the shared helper with a git argument array.
• Add [CmdletBinding(SupportsShouldProcess)]; honor -WhatIf/-Confirm.
• Fix latent bugs while porting:
  • The -IncludeUntracked flag is built as --include--untracked (double dash,
    wrong flag) — should be --include-untracked.
  • -Message is a [string] tested with -ne $null; use an
    empty/whitespace check and pass -m <message> properly.
• Prefer git stash push over the deprecated git stash save.
• Cross-platform repository path handling; drop the magic exit-code / bug branch.

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.
• README command table + [Unreleased] CHANGELOG updated.
• Deterministic Pester tests (temp repo with a dirty working tree).
• build/Test-Modules.ps1 and build/Invoke-Tests.ps1 pass.