Migrate contracts directory from npm to pnpm

- Delete package-lock.json and create pnpm-lock.yaml
- Add .npmrc with strict peer deps and frozen lockfile settings
- Update CI workflows to use pnpm instead of npm
- Update shell scripts to use pnpm instead of npm/npx
- Update documentation to reference pnpm commands
- Validate migration with successful hardhat compile

contracts/scripts/security-analysis.sh

@@ -4,7 +4,7 @@
 #
 # Prerequisites:
 #   pip install slither-analyzer mythril
-#   npm install -g solc
+#   pnpm add -g solc
 #
 # Usage:
 #   ./scripts/security-analysis.sh [--slither-only | --mythril-only]