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>

docs/public/docs.json

@@ -70,6 +70,7 @@
         "pages": [
           "context-engineering",
           "progressive-disclosure",
+          "file-read-gate",
           "smart-explore-benchmark"
         ]
       },