Remove obsolete SKIP_IDPAK_CHECK build flag

[cursor]

Summary

Removes the SKIP_IDPAK_CHECK CMake option and all associated wiring. The only runtime behavior it gated was a no-op left after FS_CheckIdPaks was removed (FS_Startup contained an empty if with a commented-out call).

Flags removed

Flag / plumbing	Notes
SKIP_IDPAK_CHECK (CMake OPTION, compile definition, presets, Android Gradle, CI env/cmake args)	No remaining C preprocessor uses after deleting the dead #ifndef block in files.c.
compile_engine.sh: skip-idpak-check / skip-paks arguments	These only forwarded -DSKIP_IDPAK_CHECK=ON; CMake already defaulted to skipping the (removed) check path in practice.

Why this is safe

• Active vs rolled-out path: Default CI/script builds already defined SKIP_IDPAK_CHECK, which excluded the #ifndef block entirely. The #else branch only ran two FS_IsBaseGame checks with an empty body—no pak validation.
• No functional regression: Filesystem init, pak loading, and dedicated-server startup are unchanged; only dead preprocessor/script noise is removed.

Behavioral parity checks

• ./scripts/compile_engine.sh vulkan (Release configure + build): passed
• ctest --output-on-failure -V from build-vk-Release (26 tests including smoke_test, renderer regression scripts, units): passed

Docs

Updated references in BUILD.md, AGENTS.md, docs/DEVELOPMENT_SETUP.md, docs/ARM_RASPBERRY_PI.md, and docs/RENDERER_CONFIDENCE.md so copy-paste CMake examples no longer mention the removed flag.