fix(stop): a registry that cannot be read is not "no row" (Issue #71)

load_state_json printed {} on a YAML/DB read error, so a registered
session was closed row-less (exit 0) and its stop workflow skipped.
stop now reads the registry strictly and exits 1 before any herdr call.
Pins of the shifted stop_session.sh lines updated.

Refs #71

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/data/skill_code_blocks.json

@@ -433,7 +433,7 @@
    "class": "c",
    "kind": "diagram",
    "evidence": [
-    "multi-agent-mux-stop/scripts/stop_session.sh:240-254"
+    "multi-agent-mux-stop/scripts/stop_session.sh:245-259"
    ],
    "mismatch": null,
    "note": "state machine"
@@ -504,8 +504,8 @@
    "class": "a",
    "evidence": [
     "multi-agent-mux-stop/scripts/stop_session.sh:101",
-    "multi-agent-mux-stop/scripts/stop_session.sh:123-188",
-    "multi-agent-mux-stop/scripts/stop_session.sh:240-244"
+    "multi-agent-mux-stop/scripts/stop_session.sh:123-193",
+    "multi-agent-mux-stop/scripts/stop_session.sh:245-249"
    ],
    "mismatch": [
     "T1: the doc offered --force-no-yaml, which no script accepts"
@@ -601,7 +601,7 @@
     "other"
    ],
    "passthrough": {
-    "1": "multi-agent-mux-stop/scripts/stop_session.sh:379"
+    "1": "multi-agent-mux-stop/scripts/stop_session.sh:384"
    }
   },
   "multi-agent-mux-create/scripts/create_session.sh": {