fix(config): store config.json under the persisted data dir (#86)

CONFIG_PATH resolved to ROOT_DIR/config.json (/app/config.json in Docker), but only
DATA_DIR (/app/data) is the mounted volume — every other artifact (DB, cookies, logs,
avatars) already lives under DATA_DIR. So on first run the default config was written
into the ephemeral image layer (never appearing in the volume), and a manually-placed
data/config.json was ignored because the bot read/wrote the root path.

- Move CONFIG_PATH to DATA_DIR/config.json so it lands in the volume and manual edits
  take effect.
- Add migrateLegacyConfig(): one-time move of an existing root-level config.json into
  the data dir, so existing local installs keep their settings (no silent reset).
- Tests for first-run persistence + the three migration cases.
- README directory tree updated to data/config.json.

Fixes #86

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/data/config.ts

@@ -1,4 +1,4 @@
-import { readFileSync, writeFileSync, mkdirSync } from "node:fs";
+import { readFileSync, writeFileSync, mkdirSync, existsSync, copyFileSync, rmSync } from "node:fs";
 import { dirname } from "node:path";
 
 export interface BotConfig {
@@ -58,3 +58,35 @@ export function saveConfig(path: string, config: BotConfig): void {
   mkdirSync(dirname(path), { recursive: true });
   writeFileSync(path, JSON.stringify(config, null, 2), "utf-8");
 }
+
+/**
+ * One-time migration for the config location fix (#86).
+ *
+ * Older versions wrote config.json to the app/repo ROOT, which is NOT inside the
+ * persisted data directory (the Docker volume is mounted at data/). That meant the
+ * file never landed in the volume on first run and a manually-placed data/config.json
+ * was ignored. config.json now lives under the data dir alongside the DB/cookies/logs.
+ *
+ * If a legacy root-level config exists and the new data-dir config does not yet exist,
+ * move it so existing local installs keep their customized settings. Best-effort:
+ * any failure is swallowed and loadConfig falls back to defaults.
+ *
+ * @returns true if a legacy config was migrated, false otherwise.
+ */
+export function migrateLegacyConfig(legacyPath: string, newPath: string): boolean {
+  try {
+    if (legacyPath === newPath) return false;
+    if (existsSync(newPath)) return false; // new location already populated — leave it
+    if (!existsSync(legacyPath)) return false; // nothing to migrate
+    mkdirSync(dirname(newPath), { recursive: true });
+    copyFileSync(legacyPath, newPath); // copy first (works across filesystems)
+    try {
+      rmSync(legacyPath);
+    } catch {
+      /* leave the legacy file if it can't be removed; the new one wins */
+    }
+    return true;
+  } catch {
+    return false;
+  }
+}