feat: add project requirements and user stories for Adiuva MVP

- Created prd.json with detailed user stories, acceptance criteria, and implementation notes for the Adiuva project.
- Updated progress.txt to reflect recent implementations of routers for checkpoints and notes, along with learnings for future iterations.
- Modified ralph.sh to change the default tool from "amp" to "claude" for improved compatibility.

scripts/ralph/ralph.sh

@@ -5,7 +5,7 @@
 set -e
 
 # Parse arguments
-TOOL="amp"  # Default to amp for backwards compatibility
+TOOL="claude"  # Default to claude for backwards compatibility
 MAX_ITERATIONS=10
 
 while [[ $# -gt 0 ]]; do