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/development-guidelines/SKILL.md

@@ -0,0 +1,107 @@
+---
+name: development-guidelines
+description: A comprehensive guide and template for establishing a unified development process and coding conventions across the team. Use when creating development guidelines and when implementing code.
+---
+
+# Development Guidelines Skill
+
+Covers the two elements needed for team development:
+1. Coding conventions for implementation (implementation-guide.md)
+2. Standardization of the development process (process-guide.md)
+
+## Prerequisites
+
+Before starting to create development guidelines, check the following:
+
+### Recommended Documents
+
+1. `docs/architecture.md` (architecture design document) - confirm the technology stack
+2. `docs/repository-structure.md` (repository structure) - confirm the directory structure
+
+The development guidelines define concrete coding conventions and a development process
+based on the project's technology stack and directory structure.
+
+## Priority of Existing Documents
+
+**Important**: If existing development guidelines are present at `docs/development-guidelines.md`,
+follow this priority order:
+
+1. **The existing development guidelines (`docs/development-guidelines.md`)** - highest priority
+   - They document project-specific conventions and processes
+   - They take precedence over this skill's guide
+
+2. **This skill's guide** - reference material
+   - ./guides/implementation.md: general-purpose coding conventions
+   - ./guides/process.md: general-purpose development process
+   - Use when there are no existing guidelines, or as a supplement
+
+**When creating new**: Refer to this skill's guides and template
+**When updating**: Update while preserving the structure and content of the existing guidelines
+
+## Output Destination
+
+Save the development guidelines you create to:
+
+```
+docs/development-guidelines.md
+```
+
+## Quick Reference
+
+### When Implementing Code
+Rules and conventions for code implementation: ./guides/implementation.md
+
+Contents:
+- TypeScript/JavaScript conventions
+- Type definitions and naming conventions
+- Function design and error handling
+- Comment conventions
+- Security and performance
+- Test code implementation
+- Refactoring techniques
+
+### When Referencing/Defining the Development Process
+Git workflow, test strategy, code review: ./guides/process.md
+
+Contents:
+- Basic principles (the importance of concrete examples, explaining rationale)
+- Git workflow rules (Git Flow branching strategy)
+- Commit messages and the PR process
+- Test strategy (pyramid and coverage)
+- The code review process
+- Quality automation
+
+### Template
+When creating development guidelines: ./template.md
+
+
+## Guide by Use Case
+
+### When Developing New Code
+1. Confirm naming conventions and coding standards in ./guides/implementation.md
+2. Confirm the branching strategy and PR handling in ./guides/process.md
+3. Write tests first (TDD)
+
+### During Code Review
+- Refer to "The Code Review Process" in ./guides/process.md
+- Check for convention violations against ./guides/implementation.md
+
+### When Designing Tests
+- "Test Strategy" in ./guides/process.md (pyramid, coverage)
+- "Test Code" in ./guides/implementation.md (implementation patterns)
+
+### When Preparing a Release
+- "Git Workflow Rules" in ./guides/process.md (policy for merging into main)
+- Confirm that commit messages follow Conventional Commits
+
+## Checklist
+
+- [ ] Coding conventions are defined with concrete examples
+- [ ] Naming conventions are clear (per language and project-specific)
+- [ ] An error handling policy is defined
+- [ ] A branching strategy is decided (Git Flow recommended)
+- [ ] Commit message conventions are clear
+- [ ] A PR template is prepared
+- [ ] Test types and coverage targets are set
+- [ ] A code review process is defined
+- [ ] A CI/CD pipeline is established