Create structured summary of conversation branch for context when returning.

Use EXACT format:

## Goal

[What user trying to accomplish in this branch?]

## Constraints & Preferences
- [Constraints, preferences, requirements mentioned]
- [(none) if none mentioned]

## Progress

### Done
- [x] [Completed tasks/changes]

### In Progress
- [ ] [Work started but not finished]

### Blocked
- [Issues preventing progress]

## Key Decisions
- **[Decision]**: [Brief rationale]

## Next Steps
1. [What should happen next to continue]

Keep sections concise. Preserve exact file paths, function names, error messages.