cli: add --dry-run to decision-log-prune

[fu351]

setup, install-hooks, uninstall-hooks and uninstall all have --dry-run. decision-log-prune (src/doberman/cli/main.py:3055-3088) deletes audit rows with no way to preview how many.

What to do

1. Add dry_run to prune_decisions (src/doberman/storage/log.py:459-480): run the same deletes, count the rows, then conn.rollback() instead of committing.
2. Add --dry-run to the command and print Would prune N row(s) (A by age, B over --max-rows).

The delete predicate doesn't change, so pending AUTH rows are still never counted.

Test (next to the prune tests in tests/integration/test_decision_log.py): the dry-run count matches what a real prune then removes, and the row count is unchanged after the dry run.