chore: initialize repository with project docs and tooling

- Add persistent docs: PRD, functional design, architecture, repository structure, development guidelines, milestones

- Add brainstorming notes (docs/ideas/)

- Configure tooling: package.json, tsconfig, eslint, prettier, vitest

- Add opencode configuration (commands, skills, agents)

- Add .gitignore for node_modules, .env, data/, backups/, build outputs

.prettierignore

@@ -0,0 +1,14 @@
+# ドキュメント・設定ディレクトリ
+.opencode/
+.steering/
+docs/
+AGENTS.md
+
+# ログファイル
+*log.json
+
+# 依存関係
+node_modules/
+
+# ビルド成果物
+dist/