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

vitest.config.ts

@@ -0,0 +1,29 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: [
+      'src/**/*.{test,spec}.{ts,tsx}',
+      'tests/**/*.{test,spec}.{ts,tsx}',
+    ],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'json', 'html'],
+      exclude: [
+        'node_modules/**',
+        'dist/**',
+        '.steering/**',
+        '**/*.config.{ts,js}',
+        '**/types/**',
+      ],
+      thresholds: {
+        branches: 80,
+        functions: 80,
+        lines: 80,
+        statements: 80,
+      },
+    },
+  },
+});