[ENHANCEMENT] Register Zoo commit-message command in VS Code SCM #286

Problem (one or two sentences)

Users need to trigger commit-message generation from the VS Code Source Control UI.

Part of #145.

Context (who is affected and when)

VS Code exposes SCM menu contribution points such as scm/title and scm/input for source-control actions.

Desired behavior (conceptual, not technical)

Zoo contributes a Generate Commit Message command for Git SCM providers.

Constraints / preferences (optional)

The command should appear only for Git SCM providers.

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)

• A Zoo command id is added for commit-message generation.
• The command is contributed in src/package.json.
• Menu entries are added for scm/title and scm/input.
• The menu entries use a Git-specific when clause.
• The command is registered by the extension and can be invoked in tests.

Proposed approach (optional)

Implement the contribution point and command registration without requiring the full generation workflow in this issue.

Trade-offs / risks (optional)

Incorrect menu contribution placement could make the command noisy or invisible in multi-SCM workspaces.