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

.devcontainer/devcontainer.json

@@ -0,0 +1,11 @@
+{
+  "name": "opencode-spec-driven-boilerplate",
+  "image": "mcr.microsoft.com/devcontainers/base:bookworm",
+  "workspaceFolder": "/workspaces/opencode-spec-driven-boilerplate",
+  "features": {
+    "ghcr.io/devcontainers/features/node:1": {
+      "version": "lts"
+    }
+  },
+  "postCreateCommand": "npm install && curl -fsSL https://opencode.ai/install | bash && ln -sf $HOME/.opencode/bin/opencode /usr/local/bin/opencode"
+}