Migrate Set-Config: set git config value with ShouldProcess

[shmuelie]

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

Goal

Add a command to set a git config value (ObjectiveGit's Set-Config), with
-Location local/global/system — modernized.

Modernize (do not port as-is)

• ObjectiveGit builds
  "git -C $Repository config --$Location $Property \"$Value\"" and runs it via
  Invoke-Expression — property/value are interpolated into the command string.
  Use the shared helper with a git argument array.
• Add [CmdletBinding(SupportsShouldProcess)]; honor -WhatIf/-Confirm
  (changes persistent config, possibly --global/--system).
• Keep the -Location ValidateSet('local','global','system'); the original's
  bespoke exit-code mapping (1/3/4) can be simplified — surface git's
  stderr via the shared helper.
• Cross-platform path handling.

Acceptance criteria

• Exported from both the .psm1 and .psd1 export lists.
• Comment-based help; SupportsShouldProcess honored.
• Property/value passed as discrete arguments (never interpolated into a command
  string); final command name uses an approved verb.
• README command table + [Unreleased] CHANGELOG updated.
• Deterministic Pester tests (temp repo; set + read back a --local value;
  assert -WhatIf writes nothing).
• build/Test-Modules.ps1 and build/Invoke-Tests.ps1 pass.