fix: resolve CHANGELOG.md path issue for Docker deployment

- Update changelog.go to use binary directory as base path
  - Uses os.Executable() instead of relative path '..'
  - Automatically adapts to local and Docker environments

- Modify server Dockerfile build context
  - Change context from ./server to project root (.)
  - Update COPY commands for new context
  - Add CHANGELOG.md to container image

- Update AGENTS.md guidelines
  - Explicitly specify relative path format for file references

This ensures the changelog API works correctly in both local development
and Docker deployment environments.

server/Dockerfile

@@ -1,5 +1,6 @@
 # Go 服务 Dockerfile
 # 多阶段构建：编译阶段 + 运行阶段
+# 构建上下文：项目根目录（docker-compose context: .）
 
 # ===== 编译阶段 =====
 FROM golang:1.24-alpine AS builder
@@ -10,11 +11,11 @@ WORKDIR /build
 ENV GOPROXY=https://goproxy.cn,direct
 
 # 先复制依赖文件，利用 Docker 缓存
-COPY go.mod go.sum ./
+COPY server/go.mod server/go.sum ./
 RUN go mod download
 
 # 复制源代码并编译
-COPY . .
+COPY server/ .
 RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o billai-server .
 
 # ===== 运行阶段 =====
@@ -35,6 +36,9 @@ ENV TZ=Asia/Shanghai
 COPY --from=builder /build/billai-server .
 COPY --from=builder /build/config.yaml .
 
+# 复制项目根目录的 CHANGELOG.md
+COPY CHANGELOG.md .
+
 # 创建必要目录
 RUN mkdir -p uploads outputs