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>

test/daemon/pql_commands_test.dart

@@ -95,13 +95,6 @@ void main() {
     expect(r.error!.kind, 'user_error');
   });
 
-  test('pql.decisions.coverage returns gaps', () async {
-    await call('pql.decisions.sync');
-    final r = await call('pql.decisions.coverage');
-    expect(r.ok, isTrue);
-    expect(r.data.containsKey('gaps'), isTrue);
-  });
-
   test('pql.tickets.board returns columns', () async {
     final r = await call('pql.tickets.board');
     expect(r.ok, isTrue);