Add --git object-store counting mode

[alyx]

Summary

• add --git as an object-store-backed counterpart to --tracked, leaving --tracked behavior unchanged
• stream clean indexed blobs through one long-lived, pipelined git cat-file --batch process per repository
• read modified, conflicted, and symlink-followed paths from the filesystem to preserve working-tree semantics
• expose the mode as git: true in configuration and document its behavior

Closes #7.

Impact

Large clean repositories can avoid opening every tracked source file individually. The existing --tracked mode remains filesystem-backed, while --git opts into Git object access. Loose objects and packed objects are handled by Git itself; failures reading an object transparently fall back to the filesystem.

Validation

• go test ./...
• go test -race ./...
• go vet ./...
• differential tests covering clean, staged, modified, untracked, subdirectory, shebang, dedup, per-file, and followed-symlink behavior
• byte-identical --tracked / --git per-file JSON reports on a temporary Git repository containing the local Go standard-library source tree: 11,408 indexed files, 1.5 MB report
• packed warm-cache sample: system CPU decreased from about 1.07s (--tracked) to 0.53s (--git); wall time increased from 0.38s to 0.95s due to object inflation and ordered streaming