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()

internal/gateway/chat.go

@@ -388,13 +388,6 @@ func (g *Gateway) handleChat(w http.ResponseWriter, r *http.Request) {
 	g.singleChat(w, ctx, cands, inner, effective, rec)
 }
 
-func normalizeModel(m string) string {
-	if isAuto(m) {
-		return ""
-	}
-	return m
-}
-
 func firstModel(p *provider.Provider) string {
 	ms := p.Models()
 	if len(ms) > 0 {