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.test.ts

@@ -1,8 +1,8 @@
 import { describe, it, expect, afterEach } from "vitest";
 import { join } from "node:path";
-import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { mkdtempSync, rmSync, writeFileSync, existsSync, readFileSync } from "node:fs";
 import { tmpdir } from "node:os";
-import { getDefaultConfig, loadConfig, saveConfig } from "./config.js";
+import { getDefaultConfig, loadConfig, saveConfig, migrateLegacyConfig } from "./config.js";
 
 describe("config", () => {
   const dirs: string[] = [];
@@ -51,4 +51,57 @@ describe("config", () => {
     expect(loaded.commandPrefix).toBe("!");
     expect(loaded.autoPauseOnEmpty).toBe(true);
   });
+
+  // --- #86: config.json must live under (and be created in) the persisted data dir ---
+
+  it("first run writes config.json into the data dir and reads it back", () => {
+    const root = makeTmpDir();
+    const dataDir = join(root, "data");
+    const configPath = join(dataDir, "config.json"); // mirrors index.ts CONFIG_PATH
+
+    // Boot sequence: load (missing -> defaults) then save.
+    const config = loadConfig(configPath);
+    saveConfig(configPath, config);
+
+    expect(existsSync(configPath)).toBe(true);
+    // A subsequent hand-edited file under the SAME persisted path is honored.
+    writeFileSync(configPath, JSON.stringify({ webPort: 9999 }), "utf-8");
+    expect(loadConfig(configPath).webPort).toBe(9999);
+  });
+
+  it("migrates a legacy root config into the data dir, preserving values", () => {
+    const root = makeTmpDir();
+    const legacyPath = join(root, "config.json");
+    const newPath = join(root, "data", "config.json");
+    writeFileSync(legacyPath, JSON.stringify({ webPort: 4242, publicUrl: "http://x" }), "utf-8");
+
+    const migrated = migrateLegacyConfig(legacyPath, newPath);
+
+    expect(migrated).toBe(true);
+    expect(existsSync(newPath)).toBe(true);
+    expect(existsSync(legacyPath)).toBe(false); // legacy moved, not duplicated
+    const loaded = loadConfig(newPath);
+    expect(loaded.webPort).toBe(4242);
+    expect(loaded.publicUrl).toBe("http://x");
+  });
+
+  it("does NOT overwrite an existing data-dir config during migration", () => {
+    const root = makeTmpDir();
+    const legacyPath = join(root, "config.json");
+    const newPath = join(root, "data", "config.json");
+    writeFileSync(legacyPath, JSON.stringify({ webPort: 1111 }), "utf-8");
+    saveConfig(newPath, { ...getDefaultConfig(), webPort: 2222 });
+
+    const migrated = migrateLegacyConfig(legacyPath, newPath);
+
+    expect(migrated).toBe(false); // new location wins, untouched
+    expect(loadConfig(newPath).webPort).toBe(2222);
+    expect(existsSync(legacyPath)).toBe(true); // legacy left intact when not migrated
+  });
+
+  it("migration is a no-op when there is no legacy config", () => {
+    const root = makeTmpDir();
+    const migrated = migrateLegacyConfig(join(root, "config.json"), join(root, "data", "config.json"));
+    expect(migrated).toBe(false);
+  });
 });