feat(plane): router-addressed start signal; credentials only in the start request

Agent Substrate's golden-snapshot boot runs the same image with the same task env,
so the runner no longer starts its command by itself: after the workspaces are
ready it waits for POST /ousast/v1/start (run/task checked, once per boot, refused
after a delivered run). The reconciler sends it through svc/atenet-router with
ate-target-actor, which never addresses the golden actor (plane/router.py), and the
bound Model's secretKey variable travels only in that body, into the child's env.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

benchmarks/measurements/2026-09-08-module-audit.json

@@ -1,7 +1,7 @@
 {
-  "modules": 130,
+  "modules": 131,
   "counts": {
-    "load_bearing": 121,
+    "load_bearing": 122,
     "standalone_capability": 9
   },
   "orphaned": [],
@@ -661,6 +661,7 @@
         "cli",
         "plane.generate",
         "plane.reconciler",
+        "plane.router",
         "plane.runner",
         "plane.tasks.verify",
         "plane.workspaces"
@@ -697,6 +698,7 @@
       "importers": [
         "plane.generate",
         "plane.reconciler",
+        "plane.router",
         "plane.runner",
         "plane.workspaces"
       ]
@@ -710,6 +712,15 @@
         "plane.workspaces"
       ]
     },
+    {
+      "module": "plane.router",
+      "classification": "load_bearing",
+      "reason": "reachable from gate, map_gate, pair_gate or the CLI",
+      "importers": [
+        "plane.reconciler",
+        "plane.runner"
+      ]
+    },
     {
       "module": "plane.runner",
       "classification": "load_bearing",