feat: migrate runtime from Node.js to Bun

- Replace better-sqlite3 with bun:sqlite (native, no native addon build)
- Change all spawn('node') to spawn('bun') for agent processes
- Update package.json scripts: node→bun, tsx→bun, npm→bun
- Add bun-types for tsc compatibility
- Add bun:sqlite→better-sqlite3 shim for vitest (tests run on Node.js)
- Update Dockerfile: install bun alongside Node.js (CLIs need Node)
- Update setup/platform.ts: getNodePath() resolves bun binary
- Remove better-sqlite3 from production dependencies (devDep only for tests)

setup/register.test.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect, beforeEach } from 'vitest';
 
-import Database from 'better-sqlite3';
+import { Database } from 'bun:sqlite';
 
 /**
  * Tests for the register step.
@@ -9,7 +9,7 @@ import Database from 'better-sqlite3';
  * apostrophe in names, .env updates.
  */
 
-function createTestDb(): Database.Database {
+function createTestDb(): Database {
   const db = new Database(':memory:');
   db.exec(`CREATE TABLE IF NOT EXISTS registered_groups (
     jid TEXT NOT NULL,
@@ -29,7 +29,7 @@ function createTestDb(): Database.Database {
 }
 
 describe('parameterized SQL registration', () => {
-  let db: Database.Database;
+  let db: Database;
 
   beforeEach(() => {
     db = createTestDb();