Collaborative PRD editing after planning phase

Summary

After an agent creates a PRD during the planning phase, allow the markdown document to be edited by multiple collaborators (other developers) before implementation begins.

Problem

Currently, when a planning agent generates a PRD (STATE.md), there's no built-in workflow for:

• Multiple developers to review and comment on the plan
• Collaborative editing of requirements before work begins
• Approval workflow from stakeholders

Proposed Solution

Add a collaborative editing phase between planning completion and implementation:

1. PRD Review Mode - After planning completes, enter a "review" state where:

• The PRD is visible in the dashboard with an edit interface
• Multiple users can suggest changes (comments/inline edits)
• Changes are tracked with attribution

1. Approval Workflow - Before implementation can start:

• Designated approvers can sign off on the PRD
• Required approvals configurable per project
• Clear visual indicator of approval status

1. Integration Options:

• Option A: Built-in markdown editor in dashboard
• Option B: Integration with external tools (Google Docs, Notion, GitHub PR-style reviews)
• Option C: Export to collaborative platform, import back when done

Acceptance Criteria

• [ ] PRD can be viewed and edited after planning phase completes
• [ ] Multiple users can make changes with attribution
• [ ] Approval workflow prevents premature implementation start
• [ ] Changes sync back to the workspace .planning/ directory
• [ ] History of changes is preserved

Priority

High - This is essential for team workflows where multiple stakeholders need input on requirements before implementation.