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/repository-structure/SKILL.md

@@ -0,0 +1,51 @@
+---
+name: repository-structure
+description: A detailed guide and template for creating a repository structure definition document. Use only when defining the repository structure.
+---
+
+# Repository Structure Definition Skill
+
+This skill is a detailed guide for defining a clear and maintainable repository structure.
+
+## Prerequisites
+
+Before you begin defining the repository structure, confirm the following:
+
+### Required documents
+
+1. `docs/product-requirements.md` (PRD)
+2. `docs/functional-design.md` (Functional Design Document)
+3. `docs/architecture.md` (Architecture Design Document)
+
+The repository structure defines a concrete directory layout that reflects the technology stack and system composition determined during architecture design.
+
+## Priority of existing documents
+
+**Important**: If an existing repository structure definition document is present at `docs/repository-structure.md`, follow the priority order below:
+
+1. **Existing repository structure definition document (`docs/repository-structure.md`)** - Highest priority
+   - Contains the project-specific directory structure
+   - Takes precedence over this skill's guide
+
+2. **This skill's guide** - Reference material
+   - Generic templates and examples
+   - Use when there is no existing definition document, or as a supplement
+
+**When creating from scratch**: Refer to this skill's template and guide
+**When updating**: Update while preserving the structure and content of the existing definition document
+
+## Output destination
+
+Save the repository structure definition document you create to the following location:
+
+```
+docs/repository-structure.md
+```
+
+## Template reference
+
+When creating the repository structure definition document, use the following template: ./template.md
+
+## Detailed guide
+
+For a more detailed creation guide, refer to the following file: ./guide.md