Adds `--staged-against <base>` CLI flag to `check.mjs` so the reviewer
agent can compare `git diff <base>...HEAD` instead of the git index.
This gives the sandcastle reviewer a CI-compatible code path that works
in its clean sandbox where `git diff --cached` may be empty.
Appends a "Library-trace check" section to `.sandcastle/reviewer.prompt.md`
instructing the reviewer to run the command before issuing a verdict.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Danijel Martinek
26bcbb7a91