docs(plugin): document thinking-only assistant message projection undercount

31% of assistant messages have reasoning parts but no text part. Their
reasoning bytes are freed when the associated tool tag drops, but are
not reflected in any tag's reasoningByteSize. This is a known
conservative undercount that may be addressed in a future pass.

Author: ualtinok

packages/plugin/src/hooks/magic-context/compartment-trigger.ts

@@ -70,6 +70,11 @@ function estimateProjectedPostDropPercentage(
 
     // 2. Heuristic auto-drop: old tool outputs outside protected tail
     // 3. Reasoning clearing: reasoning bytes on message tags between watermark and age cutoff
+    // Note: reasoning on thinking-only assistant messages (no text part) is attributed to
+    // subsequent tool tags at runtime via precedingThinkingParts, but not reflected in any
+    // tag's reasoningByteSize. This is a known conservative undercount (~31% of assistant
+    // messages are thinking-only). Those bytes are freed when the tool tag is dropped, but
+    // the projection doesn't account for them.
     const maxTag = activeTags.reduce((max, t) => Math.max(max, t.tagNumber), 0);
     if (autoDropToolAge !== undefined && protectedTags !== undefined) {
         const toolAgeCutoff = maxTag - autoDropToolAge;