docs + dev.sh: loop in README/REPORT, ./dev.sh loop runner, scan-failure exit codes

README gains the self-host loop section (flow diagram, config reference
rows for [loop]/[redmine.status_map]/[keyproxy]/[gitea], CLI flags,
status table); REPORT.md records phase 3a including the die list for the
bash stack it replaces. dev.sh grows a `loop` runner (repo bind-mounted,
key env passthrough). `harness loop --once` now exits 2 when the intake
scan itself fails so cron setups alert loudly; daemon mode still logs
and retries.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

cmd/harness/main.go

@@ -176,7 +176,12 @@ func runLoop(args []string) int {
 		DryRun:   *dryRun,
 	}); err != nil {
 		fmt.Fprintf(os.Stderr, "harness: %v\n", err)
-		return 1
+		switch {
+		case errors.Is(err, loop.ErrIntake):
+			return 2
+		default:
+			return 1
+		}
 	}
 	return 0
 }