[ENHANCEMENT] Resolve Git repository from SCM command context #287

Problem (one or two sentences)

Multi-root VS Code workspaces may have multiple Git repositories, so Zoo must target the correct repository.

Part of #145.

Context (who is affected and when)

A user can invoke the commit-message command from a specific Source Control repository or input box.

Desired behavior (conceptual, not technical)

When invoked from Source Control, Zoo finds the matching Git repository for that SCM context.

Constraints / preferences (optional)

Use the built-in VS Code Git extension API where possible and avoid guessing when the target repository is ambiguous.

Request checklist

• I've searched existing Issues and Discussions for duplicates
• This describes a specific problem with clear context and impact

Zoo Code Task Links (optional)

No response

Optional (for contributors): You can stop here if you're just proposing the improvement.

Acceptance criteria (optional)

• The built-in vscode.git extension is activated or accessed safely.
• The repository matching the SCM argument rootUri is selected when available.
• Fallback occurs only when the target is unambiguous.
• A clear error is shown when no Git repository is available.
• Unit tests mock multiple repositories and verify selection behavior.

Proposed approach (optional)

Add a small repository resolution helper used by the SCM command handler before generation or input-box mutation.

Trade-offs / risks (optional)

Wrong repository selection would write messages into the wrong commit input in multi-root workspaces.