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

@@ -0,0 +1,96 @@
+# 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}