fix: .cbmignore negation overrides built-in skip directories

[mvanhorn]

What does this PR do?

Lets a .cbmignore negation pattern (e.g. !obj/) un-skip a directory that is in the built-in ALWAYS_SKIP_DIRS set, so users can opt a normally-skipped directory back into discovery. This ships the negation-override half of #500 the maintainer described; the joint config-key default-override (#497) is deferred.

Refs #500

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

AI was used for assistance.

[DeusData]

Thanks @mvanhorn — this reviewed clean and is ready (the negation-override logic is sound and well-tested). It now conflicts with main (src/discover/discover.c). Could you rebase on latest main? I'll merge once it's conflict-free. 🙏

[DeusData]

Thanks @mvanhorn — this is clean and well-tested, and I appreciate the scope discipline (deferring the #497 half). It's now conflicting with main — could you rebase? I'll merge once it's conflict-free. 🙏

[mvanhorn]

Thanks @DeusData! Rebased onto latest main. The conflicts were in discover.c and test_discover.c where your global_gi work and this PR's cbmignore_negation_overrides both extended the same should_skip_directory/walk_dir signatures and the discover test helpers. Resolved by threading both parameters through every signature and call site, and keeping both sets of tests (your .git/info/exclude tests and the negation tests) under the unified discover_has_rel_path helper. All discover tests pass locally and the touched files are clang-formatted. Should be conflict-free now.