drop dead pql.decisions.coverage IPC command

The 'pql decisions coverage' subcommand was removed in pql 1.4.x — the
replacement is the 'coverage_gaps' field on 'pql plan status'. No
production callers used this IPC surface; only the unit test referenced
it.

Removes the IPC registration, the PqlClient helper, and the matching
test case. Net negative LOC.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/src/pql/client.dart

@@ -106,10 +106,6 @@ class PqlClient {
     return _runObject(['decisions', 'read', id]);
   }
 
-  Future<List<Map<String, Object?>>> decisionCoverage() async {
-    return _runList(['decisions', 'coverage']);
-  }
-
   Future<List<Map<String, Object?>>> ticketList({
     String? status,
     String? team,