fix: address PR review comments and add file read gate docs

Fix indentation bugs flagged in PR review (SettingsDefaultsManager,
MigrationRunner), add current date/time to file read gate timeline
so the model can judge observation recency, and add documentation
for the file read gate feature.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/sqlite/migrations/runner.ts

@@ -35,7 +35,7 @@ export class MigrationRunner {
     this.addOnUpdateCascadeToForeignKeys();
     this.addObservationContentHashColumn();
     this.addSessionCustomTitleColumn();
-this.createObservationFeedbackTable();
+    this.createObservationFeedbackTable();
     this.addSessionPlatformSourceColumn();
   }