Migrate Remove-Branch: delete local/remote branch with high-impact ShouldProcess

[shmuelie]

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

Goal

Add a command to delete a local or remote branch (ObjectiveGit's
Remove-Branch), with -Force and -Remote — 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.
• This is destructive: add [CmdletBinding(SupportsShouldProcess, ConfirmImpact='High')] and honor -WhatIf/-Confirm.
• Cross-platform repository path handling.
• Drop the magic exit-code / "Bug with ObjectiveGit" branch (helper handles it).

Acceptance criteria

• Exported from both the .psm1 and .psd1 export lists.
• Comment-based help; high-impact SupportsShouldProcess honored.
• Final command name uses an approved verb and fits the module surface.
• README command table + [Unreleased] CHANGELOG updated.
• Deterministic Pester tests (e.g. $TestDrive repo with a throwaway branch;
  assert -WhatIf does not delete).
• build/Test-Modules.ps1 and build/Invoke-Tests.ps1 pass.