ci: subtree lint scope

In #34804 the locale linter tripped over a missing LC_ALL in libmultiprocess. That was trivial to fix, and perhaps a good thing, but it raised the question to what extend we should be linting subtrees: https://github.com/bitcoin-core/libmultiprocess/pull/265#issuecomment-4136321000

Currently it's rather inconsistent; each linter has their own list of subtree exceptions, which doesn't include some old libraries (e.g. `crc32c`) and misses some new ones (e.g. `libmultiprocess`).

Should we make one list and exclude it from all linters? Or do we strive to keep the same code style in subtrees and make exceptions only as needed?

Start by comparing the subtree exception lists used by each linter, then read #34804 and the linked libmultiprocess discussion. Check how older subtrees such as crc32c and newer ones such as libmultiprocess are currently handled. Done requires an agreed policy and consistent linter behavior, but the issue does not yet specify which policy to choose.

Written by the indexing model from the issue text.