Point the Progmasoft-authored code at Progmasoft C++ Style

The contributing guide described only the style of the code derived
from Catch2. The code under src/Progmasoft/ and tests/Progmasoft/ is
not derived from it and follows Progmasoft C++ Style
(https://github.com/Progmasoft/progmasoft-cpp), the C++ guide of the
other Progmasoft projects.

A new section under "Writing code" says so, keeps the upstream style
for the Catch2-derived code so upstream changes still apply cleanly,
and records the two deviations the Catch2 API requires: the namespace
Progmasoft::Catch3, and exceptions, which report a failed assertion and
stay inside the library.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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