rule-admit

Add one retained Clippy lint or native detector to rust-doctor's catalog, end to end, with its fixtures, evidence record, corpus entry and every hard-coded counter kept in sync. Use when the user asks to admit a rule, add a lint to the catalog, ship a rule that triage retained, or says "admit clippy::x". Handles one rule per run. Not for deciding which lints to consider, which is rule-candidate.