[ENHANCEMENT] Add commit-message support prompt type #283

Problem (one or two sentences)

Generated commit messages need a default convention and a customization point.

Part of #145.

Context (who is affected and when)

Zoo already has support prompts for quick actions, and commit-message generation should reuse that system instead of introducing a separate prompt mechanism.

Desired behavior (conceptual, not technical)

Zoo has a COMMIT_MESSAGE support prompt that can format Git context into a commit-message generation request.

Constraints / preferences (optional)

The default prompt should request only the commit message and avoid extra explanation or markdown wrapping.

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)

• COMMIT_MESSAGE is added to SupportPromptType.
• A default commit-message prompt is available through supportPrompt.default.
• Prompt placeholders cover branch, recent commits, changed files, and diffs.
• Tests cover default prompt rendering and custom prompt override.

Proposed approach (optional)

Extend the existing support-prompt module and tests. Do not include SCM command or provider completion behavior in this issue.

Trade-offs / risks (optional)

The default prompt may need iteration for project conventions, so it should remain easy to override.