Add PRD JSON validation to catch syntax errors early

[brooksy4503]

Summary

• Validates PRD JSON immediately after agent generation
• Exits with clear error message if JSON is invalid
• Prevents silent failures during build phase
• Catches common LLM syntax errors like mismatched brackets

Problem

When LLMs generate PRD JSON files, they occasionally make syntax errors (e.g., using } instead of ] for array closure). This causes a cryptic error message during the build phase: "Could not parse stories from PRD", making it hard to debug.

Solution

Added JSON validation in loop.sh immediately after the agent generates the PRD. If invalid JSON is detected:

1. Shows the exact JSON parsing error with line/column number
2. Exits with a clear "ERROR: PRD JSON is invalid" message
3. Prevents proceeding to build phase with bad data

Testing

• Verified validation catches the original PRD syntax error (line 68: } instead of ])
• Valid PRD files pass with success message
• Invalid PRD files show detailed error output