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>

mempalace/onboarding.py

@@ -312,7 +312,7 @@ def _generate_aaak_bootstrap(
         ]
     )
 
-    (mempalace_dir / "aaak_entities.md").write_text("\n".join(registry_lines))
+    (mempalace_dir / "aaak_entities.md").write_text("\n".join(registry_lines), encoding="utf-8")
 
     # Critical facts bootstrap (pre-palace — before any mining)
     facts_lines = [
@@ -359,7 +359,7 @@ def _generate_aaak_bootstrap(
         ]
     )
 
-    (mempalace_dir / "critical_facts.md").write_text("\n".join(facts_lines))
+    (mempalace_dir / "critical_facts.md").write_text("\n".join(facts_lines), encoding="utf-8")
 
 
 def run_onboarding(