fix(discover): honor root .gitignore when indexing subfolders

[Bortlesboat]

Summary

• load .gitignore from the indexed path even when that path is a subfolder without its own .git/
• add a regression test that indexes repo/pkg and verifies pkg/.gitignore still excludes secret.py
• keep the .cbmignore override behavior from issue #510 out of scope for this PR

Related

• Addresses repro 1 in .gitignore (non repo root) gaps and overrides #510

Verification

• git diff --check
• focused discover-only runner built under WSL with cc ... tests/test_discover.c ... src/discover/*.c ... -> 75 passed

[DeusData]

Thanks @Bortlesboat — this reviewed clean and is ready, but it now conflicts with main (src/discover/discover.c shifted under a just-merged change). Could you rebase on latest main? One small thing to confirm while you're in there: does cbm_gitignore_load return NULL cleanly for a non-existent file? The now-unconditional load relies on that. I'll merge once it's rebased. 🙏

[DeusData]

Thanks @Bortlesboat — clean fix and a good reproduce-first test; it just needs a rebase on latest main (now conflicting). Rebase and I'll merge. 🙏