improve: adopt let→const from #12760, add regression test

Credit to @lailoo (PR #12760) for the let→const improvement and
the regression test pattern verifying consecutive compaction cycles.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/auto-reply/reply/agent-runner-memory.ts

@@ -521,7 +521,7 @@ export async function runMemoryFlushIfNeeded(params: {
         return result;
       },
     });
-    let memoryFlushCompactionCount =
+    const memoryFlushCompactionCount =
       activeSessionEntry?.compactionCount ??
       (params.sessionKey ? activeSessionStore?.[params.sessionKey]?.compactionCount : 0) ??
       0;

src/auto-reply/reply/reply-state.test.ts

@@ -347,6 +347,37 @@ describe("shouldRunMemoryFlush", () => {
     ).toBe(true);
   });
 
+  it("triggers on every compaction cycle when flush records pre-increment count (#12590)", () => {
+    const params = {
+      contextWindowTokens: 100_000,
+      reserveTokensFloor: 5_000,
+      softThresholdTokens: 2_000,
+    };
+
+    // Cycle 1: compactionCount=1, no prior flush → triggers
+    expect(
+      shouldRunMemoryFlush({ entry: { totalTokens: 95_000, compactionCount: 1 }, ...params }),
+    ).toBe(true);
+
+    // After flush records compactionCount=1, compaction increments to 2.
+    // Cycle 2: compactionCount=2, memoryFlushCompactionCount=1 → triggers
+    expect(
+      shouldRunMemoryFlush({
+        entry: { totalTokens: 95_000, compactionCount: 2, memoryFlushCompactionCount: 1 },
+        ...params,
+      }),
+    ).toBe(true);
+
+    // After flush records compactionCount=2, compaction increments to 3.
+    // Cycle 3: compactionCount=3, memoryFlushCompactionCount=2 → triggers
+    expect(
+      shouldRunMemoryFlush({
+        entry: { totalTokens: 95_000, compactionCount: 3, memoryFlushCompactionCount: 2 },
+        ...params,
+      }),
+    ).toBe(true);
+  });
+
   it("ignores stale cached totals", () => {
     expect(
       shouldRunMemoryFlush({