test(log): cover max_count early-stop at both unit and CLI level

Unit (TestStoreGaps): - test_get_commits_for_branch_max_count_stops_early: max_count=2 on a 5-commit chain returns exactly the 2 newest commits - test_get_commits_for_branch_max_count_zero_returns_all: max_count=0 (default) returns all 5 commits unchanged - test_get_commits_for_branch_max_count_larger_than_chain: max_count greater than chain length returns every commit without error

CLI integration (TestLog): - test_max_count_limits_output: muse log -n 2 on 5 commits returns exactly 2 lines showing the two most recent - test_max_count_one_returns_single_commit: muse log -n 1 returns only the HEAD commit - test_max_count_larger_than_history_returns_all: muse log -n 100 on 3 commits returns all 3