perf: latency/token experiment — plan + P0 harness

AGENTS.md

@@ -39,7 +39,7 @@ Supported subset: `graph TD|LR` nodes `A[x]` `B{decision}` `C[(db)]` edges `-->`
 Push native JSON to a running viewer — run it locally
 (`PUSH_TOKEN=dev PORT=8080 node packages/viewer/dist/server.js`, open `http://localhost:8080/w/me/`;
 the viewer ships in this repo, not the npm CLI) or point at a deployed one. Configured by two
-env vars — **if either is unset, `push`/`status` exit 3** with `Set TERMCHART_VIEWER_URL and TERMCHART_VIEWER_TOKEN.`:
+env vars — **if either is unset, `push`/`status` exit 4** with `…are not set: no termchart viewer configured.`:
 
 ```bash
 export TERMCHART_VIEWER_URL="http://localhost:8080/w/me"