feat: Phase 2.7+3 — full integration, config update, systemd unit

- Wire all goroutines in main.go: watcher, quota monitor, account
  switcher, dispatcher, janitor, and 10s state flush loop
- Add missing sections to config.example.yaml: notifications,
  dispatcher, watcher, janitor
- Add scripts/claude-failover.service systemd unit

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

scripts/claude-failover.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=Claude Failover — Session Orchestrator
+After=network.target
+
+[Service]
+Type=simple
+User=ubuntu
+ExecStartPre=/usr/bin/loginctl enable-linger ubuntu
+ExecStart=/usr/local/bin/claude-failover --config /etc/claude-failover/config.yaml
+Restart=always
+RestartSec=5
+KillMode=mixed
+TimeoutStopSec=60
+EnvironmentFile=-/etc/claude-failover/env
+
+[Install]
+WantedBy=multi-user.target