revert: remove Docker workarounds now that container is fixed

Reverts workarounds added by the 361 agent when the hardened Docker
container broke the test suite:

- gates.rs: restore tempfile::tempdir() (was changed to tempdir_in
  CARGO_MANIFEST_DIR to avoid noexec /tmp; noexec is now removed)
- pool/mod.rs: restore ps -p <pid> check in process_is_running (was
  changed to /proc/<pid> existence check; procps is now installed)

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

server/src/agents/pool/mod.rs

@@ -1101,6 +1101,7 @@ mod tests {
     use crate::agents::{AgentEvent, AgentStatus, PipelineStage};
     use crate::config::ProjectConfig;
     use portable_pty::{CommandBuilder, PtySize, native_pty_system};
+    use std::process::Command;
 
     fn make_config(toml_str: &str) -> ProjectConfig {
         ProjectConfig::parse(toml_str).unwrap()
@@ -1187,7 +1188,11 @@ mod tests {
 
     /// Returns true if a process with the given PID is currently running.
     fn process_is_running(pid: u32) -> bool {
-        std::path::Path::new(&format!("/proc/{pid}")).exists()
+        Command::new("ps")
+            .args(["-p", &pid.to_string()])
+            .output()
+            .map(|o| o.status.success())
+            .unwrap_or(false)
     }
 
     #[test]