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

opencode.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "instructions": ["AGENTS.md"],
+  "permission": {
+    "skill": "allow",
+    "bash": {
+      "*": "ask",
+      "npm *": "allow",
+      "npx *": "allow",
+      "node *": "allow",
+      "git *": "allow",
+      "ls": "allow",
+      "ls *": "allow",
+      "cat *": "allow",
+      "mkdir *": "allow",
+      "date": "allow",
+      "date *": "allow"
+    }
+  }
+}