Design and implement synthesis prompt for commit message generation

[oleander]

Task

Design the synthesis phase prompt that combines file analyses into a high-quality commit message.

Requirements

• Present file summaries clearly with visual markers (bullets/dashes)
• Emphasize capturing essential nature of changes (what and why, not just file list)
• Encourage identifying primary theme/purpose of commit
• Specify maximum character length constraint (default 72 chars)
• Use conventional commit message format
• Include examples of well-formed commit messages to guide style
• Focus on concise, informative messages without unnecessary verbosity

Success Criteria

• Generated messages focus on what changed and why
• Messages identify primary theme across multiple file changes
• Messages conform to 72-character limit (or user-configured limit)
• Quality maintained for both simple and complex commits
• Style matches git best practices

Part of #81