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.

docker-compose.yaml

@@ -27,8 +27,8 @@ services:
   # Go 后端服务
   server:
     build:
-      context: ./server
-      dockerfile: Dockerfile
+      context: .
+      dockerfile: server/Dockerfile
     container_name: billai-server
     restart: unless-stopped
     ports: