feat: Add /speckit.prd command to import and sync with existing PRD

[sbhavani]

Summary

Add a command to import an existing PRD (Product Requirements Document) and list prioritized features as candidates for /speckit.specify.

I'm happy to create a PR if this would be useful.

Use Case

Many projects already have a PRD.md that defines:

• Prioritization where MoSCoW (Must/Should/Could/Won't) is common
• Feature roadmap with status

Currently there's a gap between the PRD (product-level) and /speckit.specify (feature-level). Users manually find features in their PRD, then run
/speckit.specify separately with no linkage.

Proposed Command

/speckit.prd <path>

Import a PRD and list TODO items by priority:

/speckit.prd docs/PRD.md

Then prompt: "Which feature would you like to specify?" and feed selection into /speckit.specify.

PRD Format Support

Should parse common patterns:

<!-- MoSCoW table -->                                                                                                                                           
| Feature | Status | Description |                                                                                                                              
|---------|--------|-------------|                                                                                                                              
| CSV Export | TODO | Export results as CSV |                                                                                                                   
                                                                                                                                                                
<!-- Checkbox list -->                                                                                                                                          
- [ ] CSV export for evaluation results                                                                                                                         
- [x] Docker deployment                                                                                                                                         
                                                                                                                                                                
<!-- Roadmap section -->                                                                                                                                        
### Should Have                                                                                                                                                 
1. **CSV Export** - TODO - Export results as CSV