Fix cpplint rule discovery in Super-Linter

[Copilot]

The failing Super-Linter run was reporting 57 C++ lint errors from src/main.cpp, not because of a new regression, but because cpplint was not loading the repository's intended overrides. As a result, existing PlatformIO/Arduino-specific patterns were being linted with default cpplint rules.

• Root-level cpplint config

  • Add CPPLINT.cfg at the repository root so Super-Linter/cpplint discovers the override file during CI.
  • Keep the existing rule set unchanged; this only moves it to a location cpplint actually respects.

• Preserved repository-specific exceptions

  • Continue suppressing checks that do not fit this firmware codebase:
    • whitespace/line_length
    • build/include_subdir
    • runtime/int
    • build/include_what_you_use
    • whitespace/blank_line

• Effect

  • CI now evaluates the code against the repo’s intended cpplint policy instead of default upstream rules.

set noparent
filter=-whitespace/line_length
filter=-build/include_subdir
filter=-runtime/int
filter=-build/include_what_you_use
filter=-whitespace/blank_line