Migrate Restore-Items: restore working-tree files (argument array, ShouldProcess)

[shmuelie]

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

Goal

Add a command to restore working-tree files (ObjectiveGit's Restore-Items),
with -Force and -Source — modernized.

Modernize (do not port as-is)

• ObjectiveGit builds "git -C $Repository checkout$ExtendedCLI -- $Files" and
  runs it via Invoke-Expression — with a [string[]]$Files interpolated into
  the command string, this is a real argument-injection/quoting hazard. Use the
  shared helper with a git argument array and pass file paths as discrete
  arguments after --.
• This is destructive (overwrites working-tree files): add
  [CmdletBinding(SupportsShouldProcess)]; honor -WhatIf/-Confirm.
• Consider git restore (with --source/--staged as appropriate) over raw
  checkout -- <paths> for clearer intent.
• Cross-platform 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.
• File paths passed as an argument array (never interpolated into a command
  string); final command name uses an approved verb.
• README command table + [Unreleased] CHANGELOG updated.
• Deterministic Pester tests (temp repo; assert paths with spaces/special chars
  are handled and -WhatIf makes no changes).
• build/Test-Modules.ps1 and build/Invoke-Tests.ps1 pass.