Spin guard: derivative-work rule (AGENTS.md) + rationale (ai.md) — proactive external scan cites in commit msg; 2nd failed fix = STOP and research externally, never a 3rd guess

Project lesson cited (the 2026-08 web-box saga, 9 commits): browser sources are fixed canvases
per OBS convention — the 'gap' was a widget CSS glow, not a bug.

AGENTS.md

@@ -58,6 +58,17 @@ at a problem.
   (merged once in `App.xaml`; never duplicate per-window).
 - **Do not add comments unless the code needs them; do not expand the task queue
   on your own** — work only what the user queues.
+- **Derivative work / spin guard.** Every problem here is derivative — OBS, other
+  overlay tools, and the WPF/WebView2 ecosystems already solved it. Both sides are
+  mandatory, not advisory:
+  - **Proactive (new features):** before writing code, do a quick external scan
+    (websearch: how do OBS/CEV/overlay tools do this?) and cite the reference in the
+    commit message.
+  - **Spin guard (objectively triggered):** a SECOND failed remedy for the same
+    symptom = STOP. No third guess over the same code. Research the established
+    answer externally, cite its URL in the commit message AND in `MyMistakes.md`,
+    then code it. You do not know everything — repeated failure is a research
+    trigger, not a persistence trigger.
 - **Response style: no default planning template.** Do the work, then report
   what changed and what's next. No "Plans & Pitfalls", pros/cons tables, or
   step-by-step plans unless the user asks for a plan first (see `ai.md`).