feat: initial commit - opencode spec-driven development boilerplate

Converted from Claude Code boilerplate to opencode:
- CLAUDE.md -> AGENTS.md (opencode instructions)
- .claude/settings.json -> opencode.json (permissions schema)
- .claude/agents/ -> .opencode/agent/ (subagents with mode: subagent)
- .claude/commands/ -> .opencode/command/ (slash commands with )
- .claude/skills/ -> .opencode/skills/ (7 skills, removed allowed-tools)
- DevContainer updated to install opencode
- All .claude/ paths and Claude Code references updated

.opencode/skills/steering/templates/design.md

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+# Design Document
+
+## Architecture Overview
+
+{Describe the architecture pattern to adopt}
+
+```
+{Describe the architecture diagram in text or Mermaid}
+```
+
+## Component Design
+
+### 1. {Component name 1}
+
+**Responsibilities**:
+- {Responsibility 1}
+- {Responsibility 2}
+
+**Implementation highlights**:
+- {Points to note during implementation}
+- {Technical constraints}
+
+### 2. {Component name 2}
+
+**Responsibilities**:
+- {Responsibility 1}
+- {Responsibility 2}
+
+**Implementation highlights**:
+- {Points to note during implementation}
+- {Technical constraints}
+
+## Data Flow
+
+### {Use case name}
+```
+1. {Step 1}
+2. {Step 2}
+3. {Step 3}
+```
+
+## Error Handling Strategy
+
+### Custom Error Classes
+
+{Define the necessary error classes}
+
+### Error Handling Patterns
+
+{How errors are handled}
+
+## Test Strategy
+
+### Unit Tests
+- {Test target 1}
+- {Test target 2}
+
+### Integration Tests
+- {Test scenario 1}
+- {Test scenario 2}
+
+## Dependent Libraries
+
+{Describe any newly added libraries}
+
+```json
+{
+  "dependencies": {
+    "{library name}": "{version}"
+  }
+}
+```
+
+## Directory Structure
+
+```
+{File structure to be added or changed}
+```
+
+## Implementation Order
+
+1. {Implementation step 1}
+2. {Implementation step 2}
+3. {Implementation step 3}
+
+## Security Considerations
+
+- {Security-related points to note}
+
+## Performance Considerations
+
+- {Performance-related points to note}
+
+## Future Extensibility
+
+{A design that accounts for future extensions}

.opencode/skills/steering/templates/requirements.md

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+# Requirements
+
+## Overview
+
+{Describe the feature overview in 1-2 sentences}
+
+## Background
+
+{Why this feature is needed and what problem it solves}
+
+## Features to Implement
+
+### 1. {Feature name 1}
+- {Specific feature description}
+- {What the user will be able to do}
+
+### 2. {Feature name 2}
+- {Specific feature description}
+- {What the user will be able to do}
+
+## Acceptance Criteria
+
+### {Feature name 1}
+- [ ] {Verifiable condition 1}
+- [ ] {Verifiable condition 2}
+- [ ] {Verifiable condition 3}
+
+### {Feature name 2}
+- [ ] {Verifiable condition 1}
+- [ ] {Verifiable condition 2}
+
+## Success Metrics
+
+- {Describe quantitative goals if any}
+- {Describe qualitative goals if any}
+
+## Out of Scope
+
+The following will not be implemented in this phase:
+
+- {Feature not implemented 1}
+- {Feature not implemented 2}
+
+## Reference Documents
+
+- `docs/product-requirements.md` - Product Requirements Document
+- `docs/functional-design.md` - Functional design document
+- `docs/architecture.md` - Architecture design document

.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}