thalamus/supervisor: reap channel daemons via SIGCHLD instead of SIG_IGN

SIGCHLD=SIG_IGN at main() was auto-reaping all children in the kernel,
which broke tokio::process::Command::wait() — every tool that spawned a
subprocess (bash, mcp clients) was getting ECHILD because tokio couldn't
waitpid() on a child the kernel had already reaped.

Replace with a SIGCHLD signal handler task that reaps only PIDs listed in
channels_dir() (via waitpid(pid, WNOHANG) — ECHILD on non-child is a
harmless no-op). Tokio-spawned children aren't in PID files, so tokio's
own per-child wait paths are untouched.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/thalamus/supervisor.rs

@@ -19,6 +19,51 @@ fn channels_dir() -> PathBuf {
         .join(".consciousness/channels")
 }
 
+/// Install a SIGCHLD-driven reaper for channel daemons.
+///
+/// We can't use SIGCHLD=SIG_IGN because that makes the kernel auto-reap
+/// all children, and tokio::process::Command::wait() then returns ECHILD
+/// (breaking every tool that spawns a subprocess — bash, mcp clients, etc.).
+///
+/// Instead, on each SIGCHLD we read PID files in channels_dir() and call
+/// waitpid(pid, WNOHANG) on each. That reaps only our own zombie channel
+/// daemons; waitpid on any other PID returns ECHILD (harmless no-op).
+/// Tokio-spawned children aren't recorded in PID files, so tokio's own
+/// per-child wait paths are left free to reap them.
+pub fn start_zombie_reaper() -> tokio::task::JoinHandle<()> {
+    use tokio::signal::unix::{signal, SignalKind};
+    tokio::spawn(async move {
+        let mut sig = match signal(SignalKind::child()) {
+            Ok(s) => s,
+            Err(e) => {
+                error!("failed to install SIGCHLD handler: {}", e);
+                return;
+            }
+        };
+        while sig.recv().await.is_some() {
+            reap_channel_daemons();
+        }
+    })
+}
+
+fn reap_channel_daemons() {
+    let entries = match std::fs::read_dir(channels_dir()) {
+        Ok(e) => e,
+        Err(_) => return,
+    };
+    for entry in entries.flatten() {
+        let path = entry.path();
+        if path.extension().and_then(|s| s.to_str()) != Some("pid") {
+            continue;
+        }
+        let Ok(s) = std::fs::read_to_string(&path) else { continue };
+        let Ok(pid) = s.trim().parse::<i32>() else { continue };
+        let mut status = 0;
+        // Reaps our zombie child; ECHILD on non-child is a no-op.
+        unsafe { libc::waitpid(pid, &mut status, libc::WNOHANG); }
+    }
+}
+
 fn config_path() -> PathBuf {
     channels_dir().join("channels.json5")
 }

src/user/mod.rs

@@ -756,8 +756,10 @@ fn restore_stderr(original_fd: std::os::fd::RawFd) {
 
 #[tokio::main]
 pub async fn main() {
-    // Auto-reap child processes (channel daemons outlive the supervisor)
-    unsafe { libc::signal(libc::SIGCHLD, libc::SIG_IGN); }
+    // Reap channel-daemon zombies via a SIGCHLD handler that only touches
+    // PIDs listed in channels_dir(). Avoids SIGCHLD=SIG_IGN, which would
+    // break tokio::process::Command::wait() (kernel auto-reap → ECHILD).
+    let _reaper = crate::thalamus::supervisor::start_zombie_reaper();
 
     // Redirect stderr to pipe — logs to file and sends to channel for UI display
     let stderr_capture = redirect_stderr_to_pipe();