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/steering/templates/tasklist.md

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+# Task List
+
+## 🚨 Principle of Fully Completing Tasks
+
+**Keep working until all tasks in this file are complete**
+
+### Mandatory Rules
+- **Make every task `[x]`**
+- "Planned as a separate task due to time constraints" is forbidden
+- "Postponed because the implementation is too complex" is forbidden
+- Do not finish work while leaving incomplete tasks (`[ ]`)
+
+### Plan Only Tasks That Can Be Implemented
+- At the planning stage, list only "tasks that can be implemented"
+- Do not include "tasks that might be done in the future"
+- Do not include "tasks under consideration"
+
+### The Only Case Where Skipping a Task Is Permitted
+Skipping is possible only when one of the following technical reasons applies:
+- A change in the implementation approach made the feature itself unnecessary
+- An architecture change replaced it with a different implementation method
+- A change in dependencies made the task impossible to execute
+
+When skipping, always state the reason clearly:
+```markdown
+- [x] ~~task name~~ (unnecessary due to a change in approach: specific technical reason)
+```
+
+### If a Task Is Too Large
+- Split the task into smaller subtasks
+- Add the split subtasks to this file
+- Complete the subtasks one by one
+
+---
+
+## Phase 1: {Phase name}
+
+- [ ] {Task 1}
+  - [ ] {Subtask 1-1}
+  - [ ] {Subtask 1-2}
+
+- [ ] {Task 2}
+  - [ ] {Subtask 2-1}
+  - [ ] {Subtask 2-2}
+
+## Phase 2: {Phase name}
+
+- [ ] {Task 1}
+  - [ ] {Subtask 1-1}
+  - [ ] {Subtask 1-2}
+
+- [ ] {Task 2}
+
+## Phase 3: Quality Check and Fixes
+
+- [ ] Confirm that all tests pass
+  - [ ] `npm test`
+- [ ] Confirm that there are no lint errors
+  - [ ] `npm run lint`
+- [ ] Confirm that there are no type errors
+  - [ ] `npm run typecheck`
+- [ ] Confirm that the build succeeds
+  - [ ] `npm run build`
+
+## Phase 4: Documentation Updates
+
+- [ ] Update README.md (as needed)
+- [ ] Post-implementation retrospective (record at the bottom of this file)
+
+---
+
+## Post-Implementation Retrospective
+
+### Implementation Completion Date
+{YYYY-MM-DD}
+
+### Differences Between Plan and Actual
+
+**Points that differed from the plan**:
+- {Technical changes not anticipated at planning time}
+- {Changes in the implementation approach and the reasons}
+
+**Tasks that became newly necessary**:
+- {Tasks added during implementation}
+- {Why the addition was necessary}
+
+**Tasks skipped for technical reasons** (only when applicable):
+- {Task name}
+  - Reason for skipping: {specific technical reason}
+  - Alternative implementation: {what it was replaced with}
+
+**⚠️ Note**: Do not list tasks skipped for reasons such as "time constraints" or "difficulty" here. Completing all tasks is the principle.
+
+### Lessons Learned
+
+**Technical insights**:
+- {Technical knowledge gained through implementation}
+- {New technologies or patterns used}
+
+**Process improvements**:
+- {What went well in task management}
+- {How the steering files were leveraged}
+
+### Improvement Suggestions for Next Time
+- {Things to watch out for in the next feature addition}
+- {More efficient implementation methods}
+- {Improvements to task planning}