Add Debian packaging support with cargo-deb

- Add cargo-deb metadata to socktop and socktop_agent Cargo.toml
- Create systemd service file for socktop_agent
- Add postinst/postrm maintainer scripts for user/group management
- Create GitHub Actions workflow to build .deb packages for AMD64 and ARM64
- Add comprehensive documentation in docs/DEBIAN_PACKAGING.md
- Packages will be available as artifacts on every push
- Automatic GitHub releases for version tags

socktop_agent/debian/postinst

@@ -0,0 +1,26 @@
+#!/bin/sh
+set -e
+
+# Create socktop user and group if they don't exist
+if ! getent group socktop >/dev/null; then
+    addgroup --system socktop
+fi
+
+if ! getent passwd socktop >/dev/null; then
+    adduser --system --ingroup socktop --home /var/lib/socktop \
+            --no-create-home --disabled-password --disabled-login \
+            --gecos "Socktop Agent" socktop
+fi
+
+# Create state directory
+mkdir -p /var/lib/socktop
+chown socktop:socktop /var/lib/socktop
+chmod 755 /var/lib/socktop
+
+# Create config directory if it doesn't exist
+mkdir -p /etc/socktop
+chmod 755 /etc/socktop
+
+#DEBHELPER#
+
+exit 0