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

.gitignore

@@ -0,0 +1,46 @@
+# Temporary files
+tmp/
+
+# Node.js
+node_modules/
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# Build outputs
+dist/
+build/
+*.tsbuildinfo
+
+# Environment variables
+.env
+.env.local
+.env.*.local
+
+# Logs
+logs/
+*.log
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# Testing
+coverage/
+.nyc_output/
+
+# Steering files (task management - temporary)
+.steering/*
+!.steering/.gitkeep
+
+# Lock files (keep package-lock.json for consistency)
+# yarn.lock
+
+.opencode/opencode.local.json