fix(extraction): index nested non-submodule git repos (#193) (#217)

`codegraph init -i` from a git super-repo containing independent nested
git repositories (not submodules) reported "No files found to index":
git ls-files reports an embedded repo only as an opaque `subdir/` entry
and never lists its files. Detect embedded repos via that trailing-slash
signal and recurse `git ls-files` into each, indexing tracked + untracked
source and honoring each repo's own .gitignore.

Reported by @timxx.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>