Add production file logging with logrotate support

Both control plane and host agent now write structured slog output to
$WRENN_DIR/logs/ in addition to stderr. Log level is configurable via
LOG_LEVEL env var (default: info). SIGHUP reopens the log file so
logrotate can rotate without copytruncate.

deploy/logrotate/wrenn

@@ -0,0 +1,19 @@
+/var/lib/wrenn/logs/control-plane.log
+/var/lib/wrenn/logs/host-agent.log
+{
+    daily
+    rotate 3
+    missingok
+    notifempty
+    dateext
+    dateformat -%Y-%m-%d
+    compress
+    delaycompress
+    sharedscripts
+    postrotate
+        # Signal the processes to reopen their log files.
+        # Use SIGHUP — both binaries handle it gracefully.
+        pkill -HUP -f wrenn-cp || true
+        pkill -HUP -f wrenn-agent || true
+    endscript
+}