feat: add per-session model selection

src/auto-reply/model.ts

@@ -0,0 +1,19 @@
+export function extractModelDirective(body?: string): {
+  cleaned: string;
+  rawModel?: string;
+  hasDirective: boolean;
+} {
+  if (!body) return { cleaned: "", hasDirective: false };
+  const match = body.match(
+    /(?:^|\s)\/model(?=$|\s|:)\s*:?\s*([A-Za-z0-9_.:-]+(?:\/[A-Za-z0-9_.:-]+)?)?/i,
+  );
+  const rawModel = match?.[1]?.trim();
+  const cleaned = match
+    ? body.replace(match[0], "").replace(/\s+/g, " ").trim()
+    : body.trim();
+  return {
+    cleaned,
+    rawModel,
+    hasDirective: !!match,
+  };
+}