feat(02-01): create database connection factory

- src/db/config.ts: getDbPath() returns ~/.cw/data/cw.db with CW_DB_PATH override
- src/db/index.ts: createDatabase() factory with WAL mode and foreign keys
- drizzle.config.ts: Drizzle Kit configuration for migrations

Pattern: Factory function (not singleton) allows isolated test instances

drizzle.config.ts

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+import { defineConfig } from 'drizzle-kit';
+import { join } from 'node:path';
+import { homedir } from 'node:os';
+
+export default defineConfig({
+  schema: './src/db/schema.ts',
+  out: './drizzle',
+  dialect: 'sqlite',
+  dbCredentials: {
+    url: process.env.CW_DB_PATH ?? join(homedir(), '.cw', 'data', 'cw.db'),
+  },
+});

src/db/config.ts

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+import { mkdirSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { homedir } from 'node:os';
+
+/**
+ * Get the database path.
+ *
+ * - Default: ~/.cw/data/cw.db
+ * - Override via CW_DB_PATH environment variable
+ * - For testing, pass ':memory:' as CW_DB_PATH
+ */
+export function getDbPath(): string {
+  const envPath = process.env.CW_DB_PATH;
+  if (envPath) {
+    return envPath;
+  }
+
+  return join(homedir(), '.cw', 'data', 'cw.db');
+}
+
+/**
+ * Ensure the parent directory for the database file exists.
+ * No-op for in-memory databases.
+ */
+export function ensureDbDirectory(dbPath: string): void {
+  // Skip for in-memory database
+  if (dbPath === ':memory:') {
+    return;
+  }
+
+  const dir = dirname(dbPath);
+  mkdirSync(dir, { recursive: true });
+}

src/db/index.ts

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+import Database from 'better-sqlite3';
+import { drizzle } from 'drizzle-orm/better-sqlite3';
+import type { BetterSQLite3Database } from 'drizzle-orm/better-sqlite3';
+
+import { getDbPath, ensureDbDirectory } from './config.js';
+import * as schema from './schema.js';
+
+export type DrizzleDatabase = BetterSQLite3Database<typeof schema>;
+
+/**
+ * Create a new database connection.
+ *
+ * This is a factory function (not a singleton) to allow multiple instances
+ * for testing with isolated databases.
+ *
+ * @param path - Optional path override. Defaults to getDbPath().
+ *               Use ':memory:' for in-memory testing database.
+ * @returns Drizzle database instance with schema
+ */
+export function createDatabase(path?: string): DrizzleDatabase {
+  const dbPath = path ?? getDbPath();
+
+  // Ensure directory exists for file-based databases
+  ensureDbDirectory(dbPath);
+
+  // Create SQLite connection
+  const sqlite = new Database(dbPath);
+
+  // Enable WAL mode for better concurrent read performance
+  sqlite.pragma('journal_mode = WAL');
+
+  // Enable foreign keys (SQLite has them disabled by default)
+  sqlite.pragma('foreign_keys = ON');
+
+  // Create Drizzle instance with schema
+  return drizzle(sqlite, { schema });
+}
+
+// Re-export config utilities
+export { getDbPath, ensureDbDirectory } from './config.js';
+
+// Re-export schema and types
+export * from './schema.js';

src/db/schema.ts

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+/**
+ * Database schema for Codewalk District.
+ *
+ * Defines the four-level task hierarchy:
+ * - Initiative: Top-level project
+ * - Phase: Major milestone within initiative
+ * - Plan: Group of related tasks within phase
+ * - Task: Individual work item
+ *
+ * Schema will be defined in Task 3.
+ */
+
+// Placeholder export to allow index.ts to compile
+// Full schema defined in Task 3
+export {};