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/functional-design/SKILL.md

@@ -0,0 +1,52 @@
+---
+name: functional-design
+description: Detailed guide and template for creating a functional design document. Use only when creating a functional design document.
+---
+
+# Functional Design Document Creation Skill
+
+This skill is a detailed guide for creating high-quality functional design documents.
+
+## Prerequisites
+
+Before starting to create a functional design document, confirm the following:
+
+### docs/product-requirements.md has been created
+
+**Required**: The PRD must exist at the following location:
+
+**File path**: `docs/product-requirements.md`
+
+The functional design document details how to technically realize the requirements defined in the PRD.
+
+## Priority of Existing Documents
+
+**Important**: If an existing functional design document is present at `docs/functional-design.md`,
+follow the priority order below:
+
+1. **Existing functional design document (`docs/functional-design.md`)** - Highest priority
+   - Contains project-specific design
+   - Takes precedence over this skill's guide
+
+2. **This skill's guide** - Reference material
+   - Generic templates and examples
+   - Use when no existing design document exists, or as a supplement
+
+**When creating new**: Refer to this skill's template and guide
+**When updating**: Update while maintaining the structure and content of the existing design document
+
+## Output Destination
+
+Save the created functional design document to the following location:
+
+```
+docs/functional-design.md
+```
+
+## Template Reference
+
+When creating a functional design document, use the following template: ./template.md
+
+## Detailed Guide
+
+For a more detailed creation guide, refer to the following file: ./guide.md