docs: 删除 PLAN.md，更新 CLAUDE.md 中的引用

PLAN.md 的内容已被 CLAUDE.md、README.md 和代码覆盖：
- API 路由、功能描述、Docker 配置 → README.md
- 架构、关键模式、目录结构 → CLAUDE.md
- SQL Schema、LLM Prompts、SSE 格式 → 代码本身

更新 CLAUDE.md Development Notes，将 PLAN.md 引用改为指向实际代码文件。

CLAUDE.md

@@ -69,6 +69,6 @@ static/     → CSS (Tailwind output), JS (graph, diff-viewer, sse), vendor libs
 
 ## Development Notes
 
-- Refer to `PLAN.md` for full specification including API routes, SSE event formats, SQL schema, and prompt templates.
 - No authentication — do not expose to public internet.
-- All LLM prompts are in PLAN.md §8; model/endpoint/key are configurable via the settings page.
+- LLM prompts are in `services/generate.go` and `services/review.go`; model/endpoint/key are configurable via the settings page.
+- SSE event formats are defined in `handlers/generate.go` and `handlers/review.go` (server) and consumed by `static/js/sse.js` (client).