Remove obsolete SKIP_IDPAK_CHECK build flag

[cursor]

Summary

Removes the stale compile-time SKIP_IDPAK_CHECK option and all build/CI wiring. The only C usage was a no-op: FS_CheckIdPaks() was already removed, leaving a commented call inside #ifndef SKIP_IDPAK_CHECK.

Flags removed

Flag / option	Why safe to delete
SKIP_IDPAK_CHECK (CMake OPTION, compile definition, compile_engine.sh skip-idpak-check, presets, Android CMake args, CI CFLAGS/CXXFLAGS and -D lines)	Behavior parity: With the macro defined (previous default), the #ifndef block was skipped. With the macro undefined, the block ran but only contained a commented-out function call—no executable behavior. Default and non-default builds were therefore identical at runtime.

Behavioral parity checks performed

• cmake configure + cmake --build … --target qcommon against a fresh build tree (/tmp/idtech3-cmake-test) after removing the option—succeeds; files.c compiles.
• YAML: Parsed .github/workflows/build.yml and renderer-tier-b.yml with PyYAML after fixing macOS steps (restored env: for LIBRARY_PATH / CMAKE_PREFIX_PATH only).

Candidates not changed (low confidence / out of scope)

• USE_FLUX and other renderer USE_* CMake toggles: still gate real optional subsystems; default ON does not imply safe deletion of OFF builds.
• Runtime cvars (cl_*, r_*, etc.): no evidence in this pass that any are permanently fixed or “launched” in a removable sense without gameplay risk.

Files touched

• src/qcommon/files.c — remove dead preprocessor block in FS_Init
• CMakeLists.txt — drop OPTION + target_compile_definitions
• scripts/compile_engine.sh, CMakePresets.json, android/app/build.gradle, .github/workflows/*.yml
• Docs: BUILD.md, AGENTS.md, docs/DEVELOPMENT_SETUP.md, docs/ARM_RASPBERRY_PI.md, docs/RENDERER_CONFIDENCE.md