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/process_check.go

@@ -2,6 +2,9 @@ package e2e
 
 import (
 	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
 	"strings"
 	"time"
 )
@@ -46,7 +49,33 @@ func checkSupervisorProcesses(s *stack) error {
 		return true, nil
 	})
 	if err != nil {
-		return fmt.Errorf("%w\n==== selfpost logs ====\n%s", err, s.logs("selfpost"))
+		logs := s.logs("selfpost")
+		// #region agent log
+		ps, _ := s.compose("ps", "-a", "--format", "json")
+		inspectOut, _ := exec.Command("docker", "ps", "-a", "--filter", "name=selfpost-e2e",
+			"--format", "{{.Names}} {{.Status}} {{.ID}}").CombinedOutput()
+		dataPath := filepath.Join(s.stageDir, "data")
+		dataMode := ""
+		if fi, e := os.Stat(dataPath); e == nil {
+			dataMode = fmt.Sprintf("%04o", fi.Mode().Perm())
+		}
+		entrypointSnippet, _ := exec.Command("docker", "run", "--rm", "--entrypoint", "sh", "selfpost:e2e",
+			"-c", "grep -n 'chmod 755 /data\\|SELFPOST_HOSTNAME is not set' /usr/local/bin/entrypoint.sh | head -20").CombinedOutput()
+		agentDebugLog("H4", "process_check.go:fail", "compose selfpost not ready", map[string]any{
+			"waitErr":            err.Error(),
+			"logsLen":            len(logs),
+			"logsHead":           truncateForDebug(logs, 4000),
+			"composePs":          truncateForDebug(string(ps), 2000),
+			"dockerPs":           strings.TrimSpace(string(inspectOut)),
+			"hostDataMode":       dataMode,
+			"entrypointSnippets": strings.TrimSpace(string(entrypointSnippet)),
+		})
+		agentDebugLog("H5", "process_check.go:fail", "image entrypoint markers", map[string]any{
+			"snippet": strings.TrimSpace(string(entrypointSnippet)),
+		})
+		// #endregion
+		return fmt.Errorf("%w\n==== selfpost logs ====\n%s\n==== docker ps ====\n%s\n==== entrypoint markers ====\n%s",
+			err, logs, inspectOut, entrypointSnippet)
 	}
 	return nil
 }