refactor: remove AI governance scaffolding

[LessUp]

Summary

• remove OpenSpec/Claude/OMC governance scaffolding and duplicated AI instruction surfaces
• keep only the root CHANGELOG and simplify README, contributing guidance, and Pages navigation/content
• align public docs with the actual runtime boundary and fix the test helper constness issue found during validation

Validation

• docs: cd docs && npm ci && npm run build
• core: CC=/usr/bin/gcc CXX=/usr/bin/g++ cmake -S . -B build-systemcc -DCMAKE_BUILD_TYPE=Release -DBUILD_TESTS=ON && cmake --build build-systemcc -j8 && ctest --test-dir build-systemcc --output-on-failure --timeout 300