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/skills/steering/templates/requirements.md

@@ -0,0 +1,48 @@
+# Requirements
+
+## Overview
+
+{Describe the feature overview in 1-2 sentences}
+
+## Background
+
+{Why this feature is needed and what problem it solves}
+
+## Features to Implement
+
+### 1. {Feature name 1}
+- {Specific feature description}
+- {What the user will be able to do}
+
+### 2. {Feature name 2}
+- {Specific feature description}
+- {What the user will be able to do}
+
+## Acceptance Criteria
+
+### {Feature name 1}
+- [ ] {Verifiable condition 1}
+- [ ] {Verifiable condition 2}
+- [ ] {Verifiable condition 3}
+
+### {Feature name 2}
+- [ ] {Verifiable condition 1}
+- [ ] {Verifiable condition 2}
+
+## Success Metrics
+
+- {Describe quantitative goals if any}
+- {Describe qualitative goals if any}
+
+## Out of Scope
+
+The following will not be implemented in this phase:
+
+- {Feature not implemented 1}
+- {Feature not implemented 2}
+
+## Reference Documents
+
+- `docs/product-requirements.md` - Product Requirements Document
+- `docs/functional-design.md` - Functional design document
+- `docs/architecture.md` - Architecture design document