Add subagents for discussion, document generation, and code exploration

- Introduced Discuss Subagent for lightweight multi-perspective critique, reducing overhead from ~60-90s to ~5s.
- Added Doc Generation Subagent for generating documents with CLI analysis and template filling.
- Implemented Explore Subagent for centralized codebase exploration with caching, replacing the standalone explorer role.
- Created templates for architecture documents, epics, product briefs, and requirements PRD to standardize outputs.
- Added a review file for tracking changes and discussions.

.claude/skills/team-lifecycle/subagents/doc-generation-subagent.md

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+# Doc Generation Subagent
+
+文档生成执行引擎。由 writer 主 agent 通过 Inner Loop 调用。
+负责 CLI 多视角分析 + 模板填充 + 文件写入。
+
+## Design Rationale
+
+从 v4.0 的 writer 内联 CLI 执行，改为 subagent 隔离调用。
+好处：CLI 调用的大量 token 消耗不污染 writer 主 agent 上下文，
+writer 只拿到压缩摘要，可在多任务间保持上下文连续。
+
+## Invocation
+
+```
+Task({
+  subagent_type: "universal-executor",
+  run_in_background: false,
+  description: "Generate <doc-type>",
+  prompt: `## Document Generation: <doc-type>
+
+### Session
+- Folder: <session-folder>
+- Spec config: <spec-config-path>
+
+### Document Config
+- Type: <product-brief | requirements | architecture | epics>
+- Template: <template-path>
+- Output: <output-path>
+- Prior discussion: <discussion-file or "none">
+
+### Writer Accumulator (prior decisions)
+<JSON array of prior task summaries>
+
+### Execution Strategy
+<从 generate-doc.md 对应 doc-type 段落加载>
+
+### Output Requirements
+1. Write document to <output-path> (follow template + document-standards.md)
+2. Return JSON summary:
+{
+  "artifact_path": "<path>",
+  "summary": "<100-200字>",
+  "key_decisions": [],
+  "sections_generated": [],
+  "cross_references": [],
+  "warnings": []
+}`
+})
+```
+
+## Doc Type Strategies
+
+(直接引用 generate-doc.md 的 DRAFT-001/002/003/004 策略，不重复)
+See: roles/writer/commands/generate-doc.md
+
+## Error Handling
+
+| Scenario | Resolution |
+|----------|------------|
+| CLI 工具失败 | fallback chain: gemini → codex → claude |
+| Template 不存在 | 返回错误 JSON |
+| Prior doc 不存在 | 返回错误 JSON，writer 决定是否继续 |