[ENHANCEMENT] Add commit-message generator service #284

Problem (one or two sentences)

Zoo needs a small service that turns Git context into model output for a commit message.

Part of #145.

Context (who is affected and when)

The service should be testable without VS Code SCM and should work with Zoo's existing provider completion path.

Desired behavior (conceptual, not technical)

Given Git context and provider settings, Zoo returns cleaned commit message text.

Constraints / preferences (optional)

Keep this service independent from Source Control UI wiring.

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 service accepts Git context and returns cleaned commit-message text.
• The implementation uses singleCompletionHandler or an injected equivalent.
• Code fences, surrounding quotes, extra whitespace, and obvious wrapper text are stripped.
• Empty/no-change context returns a clear user-facing failure.
• Unit tests cover success, cleanup, provider failure, and empty context.

Proposed approach (optional)

Add a focused generator module that depends on the Git context contract and COMMIT_MESSAGE prompt contract.

Trade-offs / risks (optional)

Provider errors and model responses are variable, so response cleanup should be conservative and well tested.