test(e2e): instrument hostname gate and startup for CI debug

Capture docker inspect, /data modes, supervisorctl, and entrypoint markers
on failure (stderr DEBUG816647 lines + debug-816647.log) so release CI can
show why valid_hostname_starts / startup_processes_running die with empty logs.

Co-Authored-By: Composer <noreply@cursor.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

test/e2e/debug_agent.go

@@ -0,0 +1,48 @@
+package e2e
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// agentDebugLog appends one NDJSON line for debug-mode hypothesis testing.
+// Best-effort only — never fails the suite. Also prints to stderr so GitHub
+// Actions captures the evidence when there is no local log file.
+func agentDebugLog(hypothesisID, location, message string, data map[string]any) {
+	// #region agent log
+	payload := map[string]any{
+		"sessionId":    "816647",
+		"hypothesisId": hypothesisID,
+		"location":     location,
+		"message":      message,
+		"data":         data,
+		"timestamp":    time.Now().UnixMilli(),
+		"runId":        "pre-fix",
+	}
+	b, err := json.Marshal(payload)
+	if err != nil {
+		return
+	}
+	fmt.Fprintf(os.Stderr, "DEBUG816647 %s\n", b)
+	b = append(b, '\n')
+	paths := []string{
+		filepath.Join("..", "..", "debug-816647.log"),
+		"debug-816647.log",
+	}
+	if wd, err := os.Getwd(); err == nil {
+		paths = append([]string{filepath.Join(wd, "..", "..", "debug-816647.log")}, paths...)
+	}
+	for _, p := range paths {
+		f, err := os.OpenFile(p, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o644)
+		if err != nil {
+			continue
+		}
+		_, _ = f.Write(b)
+		_ = f.Close()
+		return
+	}
+	// #endregion
+}