Fix owner-PID lifecycle monitoring for cross-platform reliability

Two bugs caused the brainstorm server to self-terminate within 60s:

1. ownerAlive() treated EPERM (permission denied) as "process dead".
   When the owner PID belongs to a different user (Tailscale SSH,
   system daemons), process.kill(pid, 0) throws EPERM — but the
   process IS alive. Fixed: return e.code === 'EPERM'.

2. On WSL, the grandparent PID resolves to a short-lived subprocess
   that exits before the first 60s lifecycle check. The PID is
   genuinely dead (ESRCH), so the EPERM fix alone doesn't help.
   Fixed: validate the owner PID at server startup — if it's already
   dead, it was a bad resolution, so disable monitoring and rely on
   the 30-minute idle timeout.

This also removes the Windows/MSYS2-specific OWNER_PID="" carve-out
from start-server.sh, since the server now handles invalid PIDs
generically at startup regardless of platform.

Tested on Linux (magic-kingdom) via Tailscale SSH:
- Root-owned owner PID (EPERM): server survives ✓
- Dead owner PID at startup (WSL sim): monitoring disabled, survives ✓
- Valid owner that dies: server shuts down within 60s ✓

Fixes #879

RELEASE-NOTES.md

@@ -17,7 +17,7 @@ The subagent review loop (dispatching a fresh agent to review plans/specs) doubl
 
 ### Bug Fixes
 
-- **Owner-PID false positives** — the brainstorm server's `ownerAlive()` check treated EPERM (permission denied) the same as ESRCH (process not found), causing the server to self-terminate within 60 seconds whenever the owner process ran as a different user. This affected WSL (owner is a Windows process), Tailscale SSH, and any cross-user scenario. Fixed by treating EPERM as "alive". (#879)
+- **Owner-PID lifecycle fixes** — the brainstorm server's owner-PID monitoring had two bugs causing false shutdowns within 60 seconds: (1) EPERM from cross-user PIDs (Tailscale SSH, etc.) was treated as "process dead", and (2) on WSL the grandparent PID resolves to a short-lived subprocess that exits before the first lifecycle check. Fixed by treating EPERM as "alive" and validating the owner PID at startup — if it's already dead, monitoring is disabled and the server relies on the 30-minute idle timeout. This also removes the Windows/MSYS2-specific carve-out from `start-server.sh` since the server now handles it generically. (#879)
 - **writing-skills** — corrected false claim that SKILL.md frontmatter supports "only two fields"; now says "two required fields" and links to the agentskills.io specification for all supported fields (PR #882 by @arittr)
 
 ### Codex App Compatibility

skills/brainstorming/scripts/server.cjs

@@ -79,7 +79,7 @@ const URL_HOST = process.env.BRAINSTORM_URL_HOST || (HOST === '127.0.0.1' ? 'loc
 const SESSION_DIR = process.env.BRAINSTORM_DIR || '/tmp/brainstorm';
 const CONTENT_DIR = path.join(SESSION_DIR, 'content');
 const STATE_DIR = path.join(SESSION_DIR, 'state');
-const OWNER_PID = process.env.BRAINSTORM_OWNER_PID ? Number(process.env.BRAINSTORM_OWNER_PID) : null;
+let ownerPid = process.env.BRAINSTORM_OWNER_PID ? Number(process.env.BRAINSTORM_OWNER_PID) : null;
 
 const MIME_TYPES = {
   '.html': 'text/html', '.css': 'text/css', '.js': 'application/javascript',
@@ -312,8 +312,8 @@ function startServer() {
   }
 
   function ownerAlive() {
-    if (!OWNER_PID) return true;
-    try { process.kill(OWNER_PID, 0); return true; } catch (e) { return e.code === 'EPERM'; }
+    if (!ownerPid) return true;
+    try { process.kill(ownerPid, 0); return true; } catch (e) { return e.code === 'EPERM'; }
   }
 
   // Check every 60s: exit if owner process died or idle for 30 minutes
@@ -323,6 +323,19 @@ function startServer() {
   }, 60 * 1000);
   lifecycleCheck.unref();
 
+  // Validate owner PID at startup. If it's already dead, the PID resolution
+  // was wrong (common on WSL, Tailscale SSH, and cross-user scenarios).
+  // Disable monitoring and rely on the idle timeout instead.
+  if (ownerPid) {
+    try { process.kill(ownerPid, 0); }
+    catch (e) {
+      if (e.code !== 'EPERM') {
+        console.log(JSON.stringify({ type: 'owner-pid-invalid', pid: ownerPid, reason: 'dead at startup' }));
+        ownerPid = null;
+      }
+    }
+  }
+
   server.listen(PORT, HOST, () => {
     const info = JSON.stringify({
       type: 'server-started', port: Number(PORT), host: HOST,

skills/brainstorming/scripts/start-server.sh

@@ -107,12 +107,6 @@ if [[ -z "$OWNER_PID" || "$OWNER_PID" == "1" ]]; then
   OWNER_PID="$PPID"
 fi
 
-# On Windows/MSYS2, the MSYS2 PID namespace is invisible to Node.js.
-# Skip owner-PID monitoring — the 30-minute idle timeout prevents orphans.
-case "${OSTYPE:-}" in
-  msys*|cygwin*|mingw*) OWNER_PID="" ;;
-esac
-
 # Foreground mode for environments that reap detached/background processes.
 if [[ "$FOREGROUND" == "true" ]]; then
   echo "$$" > "$PID_FILE"