align pql client to the 1.5 exit-code contract; unvendor its skill

pql 1.5 returns exit 0 with an empty `[]` for zero matches (older pql
used exit 2), so the wrapper's "exit 2 = empty, not an error" carve-out
is obsolete — and risky, since a future exit 2 could mean a real error.
Any non-zero exit is now an error.

Also removed the repo's vendored .claude/skills/pql: it's generated by
`pql init` (which CONTRIBUTING already lists in setup, installing at user
scope), so a committed snapshot just shadows the current global skill
with stale content and drifts on every pql bump — this copy was a whole
version behind. Other vendored skills are clide-owned; pql's is pql's.

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

lib/src/pql/client.dart

@@ -178,8 +178,9 @@ class PqlClient {
       );
     }
     final stderr = (r.stderr as String).trim();
-    // Exit 2 = zero matches — valid empty result, not an error.
-    if (r.exitCode != 0 && r.exitCode != 2) {
+    // pql 1.5+ returns exit 0 with an empty `[]` for zero matches, so any
+    // non-zero exit is a real error (older pql used exit 2 for empty).
+    if (r.exitCode != 0) {
       throw PqlException(
         'pql ${args.first} failed',
         exitCode: r.exitCode,