docs(docker): document Cursor tarball URL source in Dockerfile.cursor

Add a comment explaining the Cursor tarball URL pattern
(downloads.cursor.com/lab/<version>/linux/<arch>/...) and that it's
scraped from the official downloads page — there's no apt/yum
package. If Cursor changes the URL scheme, the build will fail with
a curl 404 at build time, so future maintainers have a hint of
where to check.

Addresses non-blocking review comment on PR #301 from @masami-agent.

Author: brettchien

Dockerfile.cursor

@@ -11,6 +11,10 @@ FROM debian:bookworm-slim
 RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates curl procps && rm -rf /var/lib/apt/lists/*
 
 # Install Cursor Agent CLI (pinned version)
+# Tarball source: https://downloads.cursor.com/lab/<version>/linux/<arch>/agent-cli-package.tar.gz
+# URL scheme scraped from Cursor's official downloads page — no apt/yum package exists.
+# If Cursor changes this pattern, the build fails with curl 404. Monitor
+# https://cursor.com/cli or https://docs.cursor.com/cli for version/URL updates.
 ARG CURSOR_VERSION=2026.04.08-a41fba1
 RUN ARCH=$(dpkg --print-architecture) && \
     if [ "$ARCH" = "arm64" ]; then ARCH=arm64; else ARCH=x64; fi && \