[ENHANCEMENT] Add commit-message prompt settings UI #290

Problem (one or two sentences)

Teams may have custom commit-message conventions that differ from Zoo's default prompt.

Part of #145.

Context (who is affected and when)

Users already customize support prompts in Settings > Prompts.

Desired behavior (conceptual, not technical)

Users can edit and reset the commit-message prompt in the existing prompt settings UI.

Constraints / preferences (optional)

SettingsView inputs must bind through cachedState, not live extension state.

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 appears in the prompt selector.
• Editing updates cached settings and saves through the normal Settings save flow.
• Reset restores the default commit-message prompt.
• English localization is added.
• UI tests cover rendering and editing where practical.

Proposed approach (optional)

Extend the existing Prompts settings UI and reuse the support prompt editing pattern while respecting cached SettingsView state.

Trade-offs / risks (optional)

Binding directly to live extension state can cause SettingsView race conditions.