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/prd-writing/template.md

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+# Product Requirements Document
+
+## Product overview
+
+### Name
+**[Product name]** - [Subtitle]
+
+### Product concept
+- **[Concept 1]**: [Description]
+- **[Concept 2]**: [Description]
+- **[Concept 3]**: [Description]
+
+### Product vision
+[Describe the world this product aims for, in 3-5 sentences]
+
+### Purpose
+- [Purpose 1]
+- [Purpose 2]
+- [Purpose 3]
+
+## Target users
+
+### Primary persona: [Name] ([Age], [Occupation])
+- [Attribute 1: e.g., freelancer running 3-5 projects in parallel]
+- [Attribute 2: e.g., Vim/Emacs + terminal environment]
+- [Current problem: e.g., does not want to spend time on task management]
+- [Expected solution]
+- [A typical daily workflow]
+
+## Success metrics (KPIs)
+
+### Primary KPIs
+- [Metric 1]: [Target value] ([Deadline])
+  - Example: Daily active users (DAU): 100 (in 3 months)
+- [Metric 2]: [Target value] ([Deadline])
+  - Example: Task completion rate: 70% or higher
+
+### Secondary KPIs
+- [Metric 1]: [Target value] ([Deadline])
+- [Metric 2]: [Target value] ([Deadline])
+
+## Functional requirements
+
+### Core features (MVP)
+
+#### [Feature 1]
+
+**User story**:
+As a [user], I want [feature] so that [goal]
+
+**Acceptance criteria**:
+- [ ] [Criterion 1: described in a measurable form]
+- [ ] [Criterion 2: described in a measurable form]
+- [ ] [Criterion 3: described in a measurable form]
+
+**Priority**: P0 (Required)
+
+#### [Feature 2]
+
+**User story**:
+[Description]
+
+**Acceptance criteria**:
+- [ ] [Criterion 1]
+- [ ] [Criterion 2]
+
+**Priority**: P1 (Important)
+
+### CLI interface (if applicable)
+
+```bash
+# Example of basic operations
+[command] [subcommand] [options]
+# Write specific command examples
+```
+
+### Future features (Post-MVP)
+
+#### [Feature 3]
+
+[Brief description]
+
+**Priority**: P2 (Nice to have)
+
+## Non-functional requirements
+
+### Performance
+- [Requirement 1]: [Measurable standard]
+  - Example: Command execution time: within 100ms
+- [Requirement 2]: [Measurable standard]
+  - Example: Task list display: within 1 second for up to 1000 items
+
+### Usability
+- [Requirement 1]: [Measurable standard]
+  - Example: A new user can learn the basic operations within 5 minutes
+- [Requirement 2]: [Measurable standard]
+
+### Reliability
+- [Requirement 1]: [Measurable standard]
+  - Example: Zero data loss (automatic backup)
+- [Requirement 2]: [Measurable standard]
+
+### Security
+- [Requirement 1]: [Measurable standard]
+- [Requirement 2]: [Measurable standard]
+
+### Scalability
+- [Requirement 1]: [Measurable standard]
+- [Requirement 2]: [Measurable standard]
+
+## Out of scope
+
+Items explicitly considered out of scope:
+- [Item 1]
+- [Item 2]
+- [Item 3]