chore: move audit tools tests/audit, add scratch/ gitignored

Audit tools are test code (bug-finders), not scripts. Move to tests/audit/.
scripts/ now only replay-combat (live demo tool).

scratch/ = gitignored throwaway. Repro scripts, exploration, debug.

Update DEVELOPMENT.md + scripts/README to match new layout.

.gitignore

@@ -12,3 +12,4 @@ data/*.sqlite-*
 server/data/*.sqlite
 server/data/*.sqlite-*
 /data
+/scratch