test: add run-tests.sh, fix stale tests for marketplace model

- Add tests/run-tests.sh: discovers and runs all test-*.sh (including
  plugin subdirs) and the bats suite; replaces per-script pre-push calls
- Add tests/run-tests.bats: TDD coverage for run-tests.sh behaviours
- Update setup-hooks.sh: pre-push block now calls run-tests.sh
- Fix test-install.sh: remove provider adapter symlink tests (adapter
  removed in marketplace migration), guard skills loops on dir existence
- Fix test-instructions-and-docs.sh: content index checks now point to
  core/AGENTS.md (where it lives), remove ard/bug dir assertions
- Fix test-setup-hooks.sh: assert pre-push hook calls run-tests.sh

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test-setup-hooks.sh

@@ -140,6 +140,11 @@ if grep -q "check-manifests" "$PUSH_HOOK"; then
 else
   fail "pre-push hook missing check-manifests.sh call"
 fi
+if grep -q "run-tests.sh" "$PUSH_HOOK"; then
+  pass "pre-push hook calls run-tests.sh"
+else
+  fail "pre-push hook missing run-tests.sh call"
+fi
 
 # --- 6. Idempotent: second run replaces each block exactly once ---
 echo ""