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

.opencode/skills/architecture-design/template.md

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+# Architecture Design Document
+
+## Technology Stack
+
+### Languages and Runtimes
+
+| Technology | Version |
+|------|-----------|
+| Node.js | v24.11.0 |
+| TypeScript | 5.x |
+| npm | 11.x |
+
+### Frameworks and Libraries
+
+| Technology | Version | Purpose | Rationale |
+|------|-----------|------|----------|
+| [name] | [version] | [purpose] | [rationale] |
+| [name] | [version] | [purpose] | [rationale] |
+
+### Development Tools
+
+| Technology | Version | Purpose | Rationale |
+|------|-----------|------|----------|
+| [name] | [version] | [purpose] | [rationale] |
+| [name] | [version] | [purpose] | [rationale] |
+
+## Architecture Pattern
+
+### Layered Architecture
+
+```
+┌─────────────────────────┐
+│   UI layer              │ ← Accepts and displays user input
+├─────────────────────────┤
+│   Service layer          │ ← Business logic
+├─────────────────────────┤
+│   Data layer             │ ← Data persistence
+└─────────────────────────┘
+```
+
+#### UI Layer
+- **Responsibility**: Accepting user input, validation, displaying results
+- **Permitted operations**: Calling the service layer
+- **Prohibited operations**: Direct access to the data layer
+
+#### Service Layer
+- **Responsibility**: Implementing business logic, data transformation
+- **Permitted operations**: Calling the data layer
+- **Prohibited operations**: Depending on the UI layer
+
+#### Data Layer
+- **Responsibility**: Data persistence and retrieval
+- **Permitted operations**: Access to the file system and databases
+- **Prohibited operations**: Implementing business logic
+
+## Data Persistence Strategy
+
+### Storage Method
+
+| Data type | Storage | Format | Rationale |
+|-----------|----------|-------------|------|
+| [data 1] | [method] | [format] | [rationale] |
+| [data 2] | [method] | [format] | [rationale] |
+
+### Backup Strategy
+
+- **Frequency**: [e.g., every hour]
+- **Destination**: [e.g., `.backup/` directory]
+- **Generation management**: [e.g., retain the latest 5 generations]
+- **Restoration method**: [procedure]
+
+## Performance Requirements
+
+### Response Time
+
+| Operation | Target time | Measurement environment |
+|------|---------|---------|
+| [operation 1] | [time] | [environment] |
+| [operation 2] | [time] | [environment] |
+
+### Resource Usage
+
+| Resource | Limit | Rationale |
+|---------|------|------|
+| Memory | [MB] | [rationale] |
+| CPU | [%] | [rationale] |
+| Disk | [MB] | [rationale] |
+
+## Security Architecture
+
+### Data Protection
+
+- **Encryption**: [target data and method]
+- **Access control**: [file permissions, etc.]
+- **Sensitive information management**: [environment variables, config files, etc.]
+
+### Input Validation
+
+- **Validation**: [validation items]
+- **Sanitization**: [target and method]
+- **Error handling**: [secure error display]
+
+## Scalability Design
+
+### Handling Data Growth
+
+- **Expected data volume**: [e.g., 10,000 tasks]
+- **Performance degradation countermeasures**: [method]
+- **Archive strategy**: [handling of old data]
+
+### Extensibility
+
+- **Plugin system**: [presence/absence and design]
+- **Configuration customization**: [scope of what is possible]
+- **API extensibility**: [methods for future extension]
+
+## Test Strategy
+
+### Unit Tests
+- **Framework**: [framework name]
+- **Target**: [description of test target]
+- **Coverage target**: [%]
+
+### Integration Tests
+- **Method**: [test method]
+- **Target**: [description of test target]
+
+### E2E Tests
+- **Tool**: [tool name]
+- **Scenarios**: [test scenarios]
+
+## Technical Constraints
+
+### Environment Requirements
+- **OS**: [supported OS]
+- **Minimum memory**: [MB]
+- **Required disk space**: [MB]
+- **Required external dependencies**: [list]
+
+### Performance Constraints
+- [constraint 1]
+- [constraint 2]
+
+### Security Constraints
+- [constraint 1]
+- [constraint 2]
+
+## Dependency Management
+
+| Library | Purpose | Version management policy |
+|-----------|------|-------------------|
+| [name] | [purpose] | [pinned/range] |
+| [name] | [purpose] | [pinned/range] |