test: add comprehensive test coverage (35% → 58%, threshold 50%)

Add 180+ new tests across 10 test files covering previously untested modules:
- instructions_cli (0% → 100%), hooks_cli (73% → 96%), spellcheck (28% → 84%)
- palace_graph (9% → 91%), general_extractor (0% → 92%), entity_detector (0% → 69%)
- entity_registry (0% → 70%), room_detector_local (0% → 55%), layers (0% → 28%)
- onboarding (0% → 36%)

Also fixes Windows encoding bug in onboarding.py (write_text without encoding="utf-8").

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

pyproject.toml

@@ -69,7 +69,7 @@ testpaths = ["tests"]
 source = ["mempalace"]
 
 [tool.coverage.report]
-fail_under = 30
+fail_under = 50
 show_missing = true
 exclude_lines = [
     "if __name__",