docs: record recurring defect types and the mechanism covering each

The titlebar drag-region defect was fixed three times in a row, each time
only for the dialogs someone reported, and the same batch introduced two more
instances. Repeated full-table lookups and production reuse of a verification
helper showed up in several places at once for the same reason.

Add docs/quality/recurring-defects.md describing each type that has actually
recurred here, the shared mechanism or automated test that now covers it, and
the history that justifies the entry. Require reading it from AGENTS.md before
touching overlays, store lookups, retrieval entry points, cancellation, or
render-time derived values, and require extending the shared mechanism rather
than patching one reported case.

Correct the external knowledge technical design where it described the removed
configuration-snapshot checks, the shared retrieval entry point and the
unreachable probe status, and record the refactor and its validation in the
knowledge base progress log.

AGENTS.md

@@ -148,6 +148,13 @@ additional hardening beyond the actual internal-network threat model:
   repository mechanisms instead of inventing extra gates, ceremonies, or
   disabled states.
 - Add or update focused tests for behavioral changes and regressions.
+- Read [`docs/quality/recurring-defects.md`](./docs/quality/recurring-defects.md)
+  before changing overlays, store lookups, retrieval entry points, cancellation
+  behavior, or render-time derived values. It records defect types that have
+  already recurred in this repository and the shared mechanism that covers each
+  one. When fixing an instance of a listed type, extend that shared mechanism
+  instead of patching the one reported case, and add an entry when a new defect
+  type recurs.
 - After completing any functional change, inspect the affected product,
   architecture, design, feature, setup, and operational documentation and
   update every relevant document to match the implemented behavior. Treat the