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

.prettierignore

@@ -0,0 +1,14 @@
+# ドキュメント・設定ディレクトリ
+.opencode/
+.steering/
+docs/
+AGENTS.md
+
+# ログファイル
+*log.json
+
+# 依存関係
+node_modules/
+
+# ビルド成果物
+dist/