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

src/example.ts

@@ -0,0 +1,18 @@
+export function add(a: number, b: number): number {
+  return a + b;
+}
+
+export function subtract(a: number, b: number): number {
+  return a - b;
+}
+
+export function multiply(a: number, b: number): number {
+  return a * b;
+}
+
+export function divide(a: number, b: number): number {
+  if (b === 0) {
+    throw new Error('Division by zero is not allowed');
+  }
+  return a / b;
+}