Fix: allow removing $-prefixed attachments (#114)

[pico-claw]

Summary

• escape attachment keys that start with $ before building the $in predicate used by RemoveAttachments
• add a regression test that covers removing attachments with reserved keys
• document the fix in the changelog

Testing

• cmake -S . -B build -DREDUCT_CPP_USE_FETCHCONTENT=ON -DREDUCT_CPP_ENABLE_TESTS=ON && cmake --build build
• REDUCT_CPP_TOKEN_API=init-token ./build/bin/reduct-tests # reduct/store:main
• REDUCT_CPP_TOKEN_API=init-token ./build/bin/reduct-tests "~[1_19]" # reduct/store:latest (1.18.x lacks support for the new attachment tests)