Keep the failed-summary marker as the only LLM fallback

Prefix-stable compaction still uses one extractive shape on LLM
failure, but dropping the CL-6906 marker made a lossy stub look like
a real handoff summary. Restore the marker and keep the first summary
turn byte-identical across later successful passes.

Author: TheGreatAxios

src/context-compactor.test.ts

@@ -6,7 +6,6 @@ import {
   formatPlan,
   classifyTaskBoundary,
   buildLLMTurnSummary,
-  buildTurnSummary,
   COMPACTED_PREFIX,
   COMPACT_SPACER_TEXT,
   type SessionMetadata,
@@ -675,7 +674,7 @@ describe("createPruningCompactor — prefix-stable summaries (CL-6914)", () => {
     const output1 = (await compactor.apply(turns, mockStrategyCtx)).output;
     expect(firstText(output1[0]!)).toContain("Turns compacted:");
     expect(firstText(output1[0]!)).not.toContain("UNIQUE_SUCCESS_SUMMARY");
-    expect(firstText(output1[0]!)).not.toContain("Model summary unavailable");
+    expect(firstText(output1[0]!)).toContain("Model summary unavailable");
 
     const output2 = (await compactor.apply(grow(output1, 16, "ok"), mockStrategyCtx)).output;
     expect(firstText(output2[0]!)).toBe(firstText(output1[0]!));

src/session/summarizer.ts

@@ -201,8 +201,12 @@ export function createModelSummarizer(
   const maxChars = options.maxChars ?? 4000;
 
   return async (turns, ctx) => {
-    const fallback = (): string => buildTurnSummary(turns, maxChars);
+    // The marker tells the model (and anyone reading a transcript) that the
+    // compacted region is a lossy stats stub, not a real handoff summary.
+    const fallback = (reason: string): string =>
+      `[Model summary unavailable (${reason}); deterministic fallback]\n${buildTurnSummary(turns, maxChars)}`;
     try {
+
       const promptTurns: ConversationTurn[] = [
         {
           role: "system",
@@ -219,14 +223,14 @@ export function createModelSummarizer(
       const text = await complete(promptTurns, options.getSource(), signal);
       if (text.length === 0) {
         logger.warn("compaction summary call returned empty text; using deterministic fallback");
-        return fallback();
+        return fallback("empty model output");
       }
       return text.length > maxChars ? text.slice(0, maxChars) : text;
     } catch (error) {
       logger.warn("compaction summary call failed; using deterministic fallback: {error}", {
         error: error instanceof Error ? error.message : String(error),
       });
-      return fallback();
+      return fallback("summary call failed");
     }
   };
 }