docs(telemetry): expected_rebuilds is no longer the compaction signal (#607 merge-time fix)

Two comments still described the abandoned expected_rebuilds rise rule;
setPromptCache no longer reads that field for the compaction stamp.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/usage-telemetry.ts

@@ -76,7 +76,7 @@ export interface RawPromptCache {
   /** Null right after a compaction or tool-result clearing, until the next request records the rewritten size. */
   recache_tokens_if_cold?: number | null;
   misses?: number | null;
-  /** Cache rebuilds that followed a compaction or a clearing of old tool results (the compaction signal). */
+  /** Cache rebuilds that followed a compaction or a clearing of old tool results (counted at the request after one; not read for the compaction stamp). */
   expected_rebuilds?: number | null;
   /** Cache read tokens as a fraction of all input tokens this session, 0-1; null while those counts are all zero. */
   hit_ratio?: number | null;

test/usage-telemetry.test.ts

@@ -172,7 +172,7 @@ describe('parsePromptCache', () => {
     // Out-of-range or non-finite values are clamped or dropped, never stored as-is.
     expect(parsePromptCache({ prompt_cache: { warm: true, hit_ratio: 1.5 } })).toEqual({ warm: true, hitRatio: 1 });
     expect(parsePromptCache({ prompt_cache: { warm: true, hit_ratio: -0.2 } })).toEqual({ warm: true, hitRatio: 0 });
-    // expected_rebuilds is rounded and floored at 0 (it indexes the compaction rise).
+    // expected_rebuilds is rounded and floored at 0.
     expect(parsePromptCache({ prompt_cache: { warm: true, expected_rebuilds: 2.9 } })).toEqual({
       warm: true,
       expectedRebuilds: 3,