chore: remove dead code found in redundancy audit

- provider.truncate: zero callers (oneLineStr is the used superset)
- gateway.normalizeModel: zero callers
- config.resolvedAPIKey: zero callers (core.resolveSourceKey is the live equivalent)
- Stats.Records: zero callers (CSV export uses AuditRecords)
- store.containsString: zero callers
- Config.MaxConcurrent: global inflight-cap field never read; per-source
  MaxConcurrent is what actually drives semaphores. Legacy configs carrying a
  top-level max_concurrent key still load (yaml.v3 ignores unknown fields —
  verified by test).
- gui renderer esc(): zero callers; renderer uses textContent, and the embedded
  WebUI has its own esc()

cmd/gui/renderer/app.js

@@ -26,16 +26,6 @@ const state = {
 
 const $ = (s) => document.querySelector(s);
 
-function esc(s) {
-  return String(s == null ? "" : s).replace(
-    /[&<>"']/g,
-    (c) =>
-      ({ "&": "&amp;", "<": "&lt;", ">": "&gt;", '"': "&quot;", "'": "&#39;" })[
-        c
-      ],
-  );
-}
-
 function toast(msg, isErr, ms) {
   const t = $("#toast");
   t.textContent = msg;