feat(agent): implement agent execution engine and turn handling with background processing

apps/infrastructure/src/persistence/iam/session_lock_repo.rs

@@ -75,17 +75,36 @@ impl SessionLockRepository for FileSystemSessionLockRepository {
             Ok(p) => p,
             Err(_) => return false,
         };
+
+        // Resolve our own executable path once.
+        let self_exe = match std::fs::read_link("/proc/self/exe") {
+            Ok(exe) => exe,
+            Err(_) => return false,
+        };
+
+        let proc_exe = std::path::PathBuf::from(format!("/proc/{pid}/exe"));
+
+        // Phase 1: read /proc/<pid>/exe and compare with self_exe.
+        let target = match std::fs::read_link(&proc_exe) {
+            Ok(t) => t,
+            Err(_) => return false,
+        };
+        if target != self_exe {
+            return false;
+        }
+
+        // Phase 2: verify the process is still alive.
+        // SAFETY: `libc::kill(pid, 0)` does not send a signal; it only checks
+        // whether the process exists and the caller has permission to signal it.
         if unsafe { libc::kill(pid_signed, 0) != 0 } {
             return false;
         }
-        let proc_exe = std::path::PathBuf::from(format!("/proc/{pid}/exe"));
-        if let Ok(target) = std::fs::read_link(&proc_exe) {
-            if let Ok(exe) = std::env::current_exe() {
-                if target != exe {
-                    return false;
-                }
-            }
+
+        // Phase 3: re-check /proc/<pid>/exe to detect PID reuse between
+        // Phase 1 and Phase 2.
+        match std::fs::read_link(&proc_exe) {
+            Ok(recheck) if recheck == self_exe => true,
+            _ => false,
         }
-        true
     }
 }