Initial implementation of GitHub download proxy service

- Add flexible URL template configuration via environment variables
- Implement automatic repository path parsing and extraction
- Add 404 response handling for all non-200 status codes
- Support both SecNex and GitHub URL formats through URL template
- Include comprehensive documentation and Docker support
- Add proper Go project structure with .gitignore

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,17 @@
+FROM golang:1.25.3-alpine3.22 AS builder
+
+WORKDIR /app
+
+COPY ./app/go.mod ./app/go.sum ./
+
+RUN go mod download && go mod verify
+
+COPY ./app ./.
+
+RUN go build -o download .
+
+FROM alpine:latest AS runner
+
+COPY --from=builder /app/download /app/download
+
+CMD ["/app/download"]