Add comprehensive timeouts to all blocking system commands

Fixes random host disconnections caused by blocking operations preventing timely ZMQ packet transmission.

Changes:
- Add run_command_with_timeout() wrapper using tokio for async command execution
- Apply 10s timeout to smartctl (prevents 30+ second hangs on failing drives)
- Apply 5s timeout to du, lsblk, systemctl list commands
- Apply 3s timeout to systemctl show/is-active, df, ip commands
- Apply 2s timeout to hostname command
- Use system 'timeout' command for sync operations where async not needed

Critical fixes:
- smartctl: Failing drives could block for 30+ seconds per drive
- du: Large directories (Docker, PostgreSQL) could block 10-30+ seconds
- systemctl/docker: Commands could block indefinitely during system issues

With 1-second collection interval and 10-second heartbeat timeout, any blocking operation >10s causes false "host offline" alerts. These timeouts ensure collection completes quickly even during system degradation.

agent/src/collectors/nixos.rs

@@ -43,8 +43,8 @@ impl NixOSCollector {
         match fs::read_to_string("/etc/hostname") {
             Ok(hostname) => Some(hostname.trim().to_string()),
             Err(_) => {
-                // Fallback to hostname command
-                match Command::new("hostname").output() {
+                // Fallback to hostname command (with 2 second timeout)
+                match Command::new("timeout").args(["2", "hostname"]).output() {
                     Ok(output) => Some(String::from_utf8_lossy(&output.stdout).trim().to_string()),
                     Err(_) => None,
                 }