add Linux systemd service and DNS configuration

Linux:
- numa install: backs up /etc/resolv.conf, sets nameserver to 127.0.0.1
- numa uninstall: restores original /etc/resolv.conf from backup
- numa service start: installs systemd unit, enables + starts
- numa service stop: stops, disables, removes unit file
- numa service status: shows systemctl status

macOS: launchd plist (already working)

Both platforms: Restart=always / KeepAlive=true for crash recovery.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

com.numa.dns.plist

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>Label</key>
+    <string>com.numa.dns</string>
+    <key>ProgramArguments</key>
+    <array>
+        <string>/usr/local/bin/numa</string>
+    </array>
+    <key>RunAtLoad</key>
+    <true/>
+    <key>KeepAlive</key>
+    <true/>
+    <key>StandardOutPath</key>
+    <string>/usr/local/var/log/numa.log</string>
+    <key>StandardErrorPath</key>
+    <string>/usr/local/var/log/numa.log</string>
+</dict>
+</plist>