feat: Resumable Download Helper and Manifest Preservation

- Route downloads through local cache directory
- Automatically resume interrupted downloads from the byte offset
- `setup_uninstaller_context` checks if a fail had happened. If yes then
  CLI preserves existing manifest instead of wiping it.
- Interruption Signal Traps and Prompts

installers/install_agy.sh

@@ -96,7 +96,7 @@ install_agy() {
     fi
 
     log_info "Downloading release package..."
-        curl -fsSL "$url" -o "$staging_payload"
+    download_file "$url" "$staging_payload"
 
     # Verify SHA512 Checksum
     local actual_hash