feat(data): avatar file store helper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/data/avatars.ts

@@ -0,0 +1,43 @@
+import { mkdirSync, writeFileSync, readFileSync, rmSync, readdirSync, existsSync } from "node:fs";
+import { join } from "node:path";
+
+const MIME_TO_EXT: Record<string, string> = {
+  "image/png": "png",
+  "image/jpeg": "jpg",
+  "image/webp": "webp",
+};
+
+export interface AvatarStore {
+  /** Returns the relative path written (e.g. "bot-1.png"). */
+  write(botId: string, mime: string, buffer: Buffer): string;
+  read(relPath: string): Buffer | null;
+  remove(relPath: string): void;
+  getDir(): string;
+}
+
+export function createAvatarStore(dir: string): AvatarStore {
+  if (!existsSync(dir)) mkdirSync(dir, { recursive: true });
+  return {
+    write(botId, mime, buffer) {
+      const ext = MIME_TO_EXT[mime];
+      if (!ext) throw new Error(`unsupported avatar MIME: ${mime}`);
+      for (const name of readdirSync(dir)) {
+        if (name.startsWith(`${botId}.`)) rmSync(join(dir, name), { force: true });
+      }
+      const rel = `${botId}.${ext}`;
+      writeFileSync(join(dir, rel), buffer);
+      return rel;
+    },
+    read(relPath) {
+      const full = join(dir, relPath);
+      if (!existsSync(full)) return null;
+      return readFileSync(full);
+    },
+    remove(relPath) {
+      rmSync(join(dir, relPath), { force: true });
+    },
+    getDir() {
+      return dir;
+    },
+  };
+}