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/command/review-docs.md

@@ -0,0 +1,64 @@
+---
+description: Run a detailed document review using a subagent
+agent: build
+---
+
+# Document Review
+
+**Document path**: `$ARGUMENTS` (e.g. `/review-docs docs/product-requirements.md`)
+
+## How to Run
+
+```
+opencode
+> /review-docs docs/product-requirements.md
+```
+
+## Procedure
+
+### Step 1: Verify the Document Exists
+
+Check whether the specified document exists.
+
+### Step 2: Launch the doc-reviewer Subagent
+
+Launch the doc-reviewer subagent to run the review:
+
+Use the Task tool to launch the doc-reviewer subagent:
+- subagent_type: "doc-reviewer"
+- description: "Document detailed review"
+- prompt: "Please review $ARGUMENTS in detail.\n\nEvaluate it from the following perspectives:\n1. Completeness: Are all required items included?\n2. Specificity: Are there any ambiguous expressions?\n3. Consistency: Is it consistent with other documents?\n4. Measurability: Are success metrics measurable? (for a PRD)\n\nPlease produce a review report."
+
+### Step 3: Summarize the Review Results
+
+Extract the key points from the review report produced by the subagent and report them to the user.
+
+## Output Format
+
+```markdown
+# Document Review Results
+
+## Document: [file name]
+
+### Key Improvements
+
+1. [Improvement 1] (Priority: High/Medium/Low)
+2. [Improvement 2] (Priority: High/Medium/Low)
+3. [Improvement 3] (Priority: High/Medium/Low)
+
+### Overall Rating
+
+[1-5]/5
+
+### Next Actions
+
+- [Recommended action 1]
+- [Recommended action 2]
+
+For the full report, refer to the subagent's output.
+```
+
+## Notes
+
+- The review involves detailed analysis and may take a few minutes.
+- The subagent runs in an independent context, so it does not consume the main agent's context.