Rename shadow-named tests and helpers to manual-commit names

The TestShadow_* and TestShadowStrategy_* tests exercise the manual-commit strategy as a whole, and GitCommitWithShadowHooks(AsAgent) simply runs a commit through the installed hooks. Renaming them to TestManualCommit_* and GitCommitWithHooks(AsAgent) up front keeps the behaviour change that follows reviewable: the later diffs touch these tests only where what they assert changes.

No test logic changes. The integration shard script drops its TestShadow_* patterns; the renamed tests already land in the same shard through TestManualCommit_*.