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.

AGENTS.md

@@ -120,4 +120,4 @@ Upload: ZIP/XLSX → Extract → Convert UTF-8 CSV → Detect bill type → Dedu
 - **Dependencies:** Check `package.json`/`go.mod`/`requirements.txt` before adding new packages
 - **Tests:** Always run relevant test suite before committing changes
 - **Git commits:** Provide clear messages explaining the "why" of changes
-- **File references:** Use `file_path:line_number` format when mentioning code locations
+- **File references:** Use relative `file_path:line_number` format (e.g., `server/handler/changelog.go:12`) when mentioning code locations