Migrate Set-Branch: switch/checkout branch with ShouldProcess

[shmuelie]

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

Goal

Add a command to switch/checkout a branch (ObjectiveGit's Set-Branch), with
-CreateNew, -Force, and -Track behavior — modernized.

Modernize (do not port as-is)

• ObjectiveGit assembles a command string and runs it via Invoke-Expression
  ("git -C $Repository checkout$ExtendedCLI $Branch"). Use the shared helper
  with a git argument array instead.
• Add [CmdletBinding(SupportsShouldProcess)] and honor -WhatIf/-Confirm
  (this changes working-tree state, and -Force can discard local changes).
• Consider aligning with git switch / git switch -c semantics rather than
  raw checkout where it reads more clearly; keep the flag surface intuitive.
• Cross-platform repository path handling.
• Drop the magic exit-code / "Bug with ObjectiveGit" branch (handled by the
  helper).

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 existing module surface
  (name open to discussion).
• README command table + [Unreleased] CHANGELOG updated.
• Deterministic Pester tests.
• build/Test-Modules.ps1 and build/Invoke-Tests.ps1 pass.