test(pin_drift): raise line coverage to 96% with fully mocked GitHub API tests

Add deterministic mocked tests covering the uncovered branches in the
GitHub fan-out path: no-default-branch (NOT_FOUND error), SHA-at-HEAD
fast-path, stale pin with countBehind commits, catch block wrapping
classifyError, tag-pinned ref bypassing SHA prefix check, grep option
grepMatches, VALIDATION error for missing localPath, and .gitmodules
parsing for both not_github skip and ls_tree_failed paths.

Also covers line 99 in parseGoMod (replace + pseudo-version path)
via a direct parseGoMod call with a matching fixture.

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

Author: AlbinoGeek