debug: add PTY spawn diagnostics for Session: None investigation

When an agent CLI exits without creating a session, we now log:
- Number of prior sessions and total session log bytes
- Child process exit status (exit code or signal)
- Explicit SESSION NONE warning with context

This will help diagnose whether the fatal runtime error
(output.write assertion) correlates with accumulated sessions,
budget exhaustion, or something else.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/agents/pty.rs

@@ -209,7 +209,22 @@ fn run_agent_pty_blocking(
     cmd.env_remove("CLAUDECODE");
     cmd.env_remove("CLAUDE_CODE_ENTRYPOINT");
 
-    slog!("[agent:{story_id}:{agent_name}] Spawning {command} in {cwd} with args: {args:?}");
+    // Count existing session files for this worktree to detect budget exhaustion.
+    let session_dir = format!(
+        "/home/huskies/.claude/projects/-workspace--huskies-worktrees-{}/",
+        story_id.replace('_', "-").replace('.', "-")
+    );
+    let session_count = std::fs::read_dir(&session_dir)
+        .map(|d| d.filter(|e| e.as_ref().map(|e| e.path().extension().is_some_and(|ext| ext == "jsonl")).unwrap_or(false)).count())
+        .unwrap_or(0);
+    let session_bytes: u64 = std::fs::read_dir(&session_dir)
+        .map(|d| d.filter_map(|e| e.ok()).filter(|e| e.path().extension().is_some_and(|ext| ext == "jsonl")).filter_map(|e| e.metadata().ok()).map(|m| m.len()).sum())
+        .unwrap_or(0);
+
+    slog!(
+        "[agent:{story_id}:{agent_name}] Spawning {command} in {cwd} with args: {args:?} \
+         (prior_sessions={session_count}, session_log_bytes={session_bytes})"
+    );
 
     let mut child = pair
         .slave
@@ -429,7 +444,19 @@ fn run_agent_pty_blocking(
     }
 
     let _ = child.kill();
-    let _ = child.wait();
+    let wait_result = child.wait();
+    match &wait_result {
+        Ok(status) => {
+            slog!(
+                "[agent:{story_id}:{agent_name}] Child exited: {status:?}"
+            );
+        }
+        Err(e) => {
+            slog!(
+                "[agent:{story_id}:{agent_name}] Child wait error: {e}"
+            );
+        }
+    }
 
     // Wait for the reader thread to finish so it releases the cloned PTY
     // master fd before we return.  Without this, the next PTY spawn for the
@@ -438,6 +465,17 @@ fn run_agent_pty_blocking(
         slog!("[agent:{story_id}:{agent_name}] Reader thread panicked: {e:?}");
     }
 
+    // Log whether session was created — Session: None indicates CLI died
+    // before emitting any events (possible causes: rate limit, budget
+    // exhaustion, PTY write failure, CLI crash).
+    if session_id.is_none() {
+        slog_warn!(
+            "[agent:{story_id}:{agent_name}] SESSION NONE: CLI exited without creating a session. \
+             Check for 'fatal runtime error' in agent logs. \
+             prior_sessions={session_count}, session_log_bytes={session_bytes}"
+        );
+    }
+
     slog!(
         "[agent:{story_id}:{agent_name}] Done. Session: {:?}",
         session_id