feat(challenges): 新增挑战模块与详情页，优化标签栏布局

- 新增挑战列表页 `app/(tabs)/challenges.tsx`，展示热门挑战卡片
- 新增挑战详情页 `app/challenges/[id].tsx`，支持排行榜、分享与参与
- 在标签栏中新增“挑战”入口，替换原有“发现”与“AI”页
- 调整标签栏间距与圆角，适配新布局
- 新增挑战相关路由常量 `TAB_CHALLENGES`
- 迁移 `coach.tsx` 与 `explore.tsx` 至根目录，保持结构清晰

AGENTS.md

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+# Repository Guidelines
+
+## Project Structure & Module Organization
+- `app/` holds Expo Router screens; tab flows live in `app/(tabs)/`, while modal or detail pages sit alongside feature folders.  
+- Shared UI and domain logic belong in `components/`, `services/`, and `utils/`; Redux state is organized per feature under `store/`.  
+- Native iOS code (HealthKit bridge, widgets, quick actions) resides in `ios/`; design and process docs are tracked in `docs/`.  
+- Assets, fonts, and icons live in `assets/`; keep new media optimized and referenced via `@/assets` aliases.
+
+## Build, Test, and Development Commands
+- `npm run ios` / `npm run ios-device` – builds and runs the prebuilt iOS app in Simulator or on a connected device.
+- `npm run reset-project` – clears caches and regenerates native artifacts; use after dependency or native module changes.
+
+## Coding Style & Naming Conventions
+- TypeScript with React hooks is standard; use functional components and keep state in Redux slices if shared.  
+- Follow ESLint (`eslint-config-expo`) and default Prettier formatting (2 spaces, trailing commas, single quotes).  
+- Name components in `PascalCase`, hooks/utilities in `camelCase`, and screen files with kebab-case (e.g., `ai-posture-assessment.tsx`).  
+- Co-locate feature assets, styles, and tests to simplify maintenance.
+
+## Testing Guidelines
+- Automated tests are minimal; add Jest + React Native Testing Library specs under `__tests__/` or alongside modules when adding complex logic.  
+- For health and native bridges, include reproduction steps and Simulator logs in PR descriptions.  
+- Always run linting and verify critical flows on an iOS simulator (HealthKit requires a real device for full validation).
+
+## Commit & Pull Request Guidelines
+- Prefer Conventional Commit prefixes (`feat`, `fix`, `chore`, etc.) with optional scope: `feat(water): 支持自定义提醒`. Keep summaries under 80 characters.  
+- Group related changes; avoid bundling unrelated features and formatting in one commit.  
+- PRs should describe the problem, solution, test evidence (commands run, screenshots, or screen recordings), and note any iOS-specific setup.  
+- Link to Linear/Jira issues where relevant and request review from feature owners or the iOS platform team.
+
+## iOS Integration Notes
+- HealthKit, widgets, and quick actions depend on native modules: update `ios/` and re-run `npm run ios` after modifying Swift or entitlement files.  
+- Keep App Group IDs, bundle identifiers, and signing assets consistent with `app.json` and `ios/digitalpilates.xcodeproj`; coordinate credential changes with release engineering.