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/memory.rs

@@ -105,12 +105,12 @@ impl MemoryCollector {
             return Ok(());
         }
 
-        // Get usage data for all tmpfs mounts at once using df
-        let mut df_args = vec!["df", "--output=target,size,used", "--block-size=1"];
+        // Get usage data for all tmpfs mounts at once using df (with 3 second timeout)
+        let mut df_args = vec!["3", "df", "--output=target,size,used", "--block-size=1"];
         df_args.extend(tmpfs_mounts.iter().map(|s| s.as_str()));
 
-        let df_output = std::process::Command::new(df_args[0])
-            .args(&df_args[1..])
+        let df_output = std::process::Command::new("timeout")
+            .args(&df_args[..])
             .output()
             .map_err(|e| CollectorError::SystemRead {
                 path: "tmpfs mounts".to_string(),