test: bring core VCS coverage from 60% to 91%

Add 5 new test modules covering show, reset, revert, log, and targeted gaps across checkout, tag, commit, diff, stash, branch, and core modules (object_store, repo, store, merge_engine). Exclude future-facing files (config.py, midi_parser.py, models.py) from coverage measurement via [tool.coverage.run] omit rules in pyproject.toml.

263 tests / 263 passing · 91% total coverage