feat: file-based audio testing + Hetzner build scripts

CLI modes:
- --send-tone <secs>: send 440Hz test tone (no mic needed)
- --record <file.raw>: save received audio to raw PCM file
- --help: usage info
- Combine: --send-tone 10 --record out.raw

Raw PCM format: 48kHz mono s16le
Play with: ffplay -f s16le -ar 48000 -ac 1 out.raw

Build scripts:
- scripts/build-linux.sh: Hetzner VPS build with auto-cleanup
- scripts/cleanup-builder.sh: kill stale builders

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/cleanup-builder.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+set -euo pipefail
+# Clean up any wzp-builder servers left running
+echo "Looking for wzp-builder servers..."
+hcloud server list -o noheader | grep wzp-builder | while read -r line; do
+  id=$(echo "$line" | awk '{print $1}')
+  name=$(echo "$line" | awk '{print $2}')
+  echo "  Deleting $name (id=$id)..."
+  hcloud server delete "$id"
+done
+echo "Done."