Feature: Ralph PRD support incomplete - missing plan creation and phase continuation

[Milofax]

Problem

The prp-ralph.md documentation mentions PRD support (lines 35, 59-66):

| Ends with `.prd.md` | Valid - will select next phase |

### 1.4 If PRD File - Select Next Phase
If input is a `.prd.md` file:
1. Read the PRD
2. Parse Implementation Phases table
3. Find first phase with `Status: pending` where dependencies are `complete`
4. Report which phase will be executed
5. Note: The loop will create and execute a plan for this phase

However, the implementation is incomplete:

1. No plan creation instruction: Step 5 says "will create and execute a plan" but there's no instruction HOW to create the plan (e.g., call /prp-plan)
2. No PRD update after completion: After COMPLETE, Ralph doesn't update the PRD's phase status from pending → complete
3. No automatic continuation: Ralph doesn't automatically start the next pending phase after completing one

Current Behavior

• ralph .plan.md → executes plan ✓
• ralph .prd.md → finds next phase, but then... nothing? ❌

Expected Behavior

When started with a PRD file, Ralph should:

1. Find next pending phase (already documented)
2. Create a plan for that phase (missing)
3. Execute the plan until COMPLETE
4. Update PRD - mark phase as complete (missing)
5. Check for next phase - continue or exit (missing)

Proposed Addition

Add section 1.5 to prp-ralph.md:

### 1.5 If PRD File - Create and Execute Plan

After identifying the next phase (1.4):

1. Extract phase description from PRD
2. Create plan: Use `/prp-plan "{phase description}"` 
3. Wait for plan file creation
4. Continue execution with the new `.plan.md` file
5. After COMPLETE:
   - Update PRD: Set phase status to `complete`
   - Check for next pending phase with satisfied dependencies
   - If found: Repeat from step 1
   - If none: Output final completion message

Context

This was discovered while using Ralph in production. The original Ralph Wiggum technique by Geoffrey Huntley (while :; do cat PROMPT.md | claude-code ; done) is designed for single-task loops. The PRD extension is a great idea but currently incomplete.

Impact

Users who start Ralph with a PRD file expecting automatic multi-phase execution will be confused when it stops after identifying the first phase without actually doing anything.