fix: exclude docs/plans/* from PR size check

Planning docs are legitimately large (700+ lines) but should not fail
the PR size gate. Add docs/plans/* to DEFAULT_EXCLUDED_PATTERNS.

Implements: DEVX-166

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/unit/test_check_pr_size.py

@@ -26,6 +26,12 @@ class TestIsExcluded:
     def test_excludes_readme(self) -> None:
         assert is_excluded("README.md", ["README.md"])
 
+    def test_excludes_plans_glob(self) -> None:
+        assert is_excluded("docs/plans/sso-bridge-full-extraction.md", ["docs/plans/*"])
+
+    def test_does_not_exclude_specs(self) -> None:
+        assert not is_excluded("docs/specs/DEVX-165.md", ["docs/plans/*"])
+
 
 class TestCheckSize:
     def test_under_limits_passes(self) -> None: