fix: refresh session-store cache when file size changes within same mtime tick

The session-store cache used only mtime for invalidation. In fast CI
runs (especially under bun), test writes to the session store can
complete within the same filesystem mtime granularity (~1s on HFS+/ext4),
so the cache returns stale data. This caused non-deterministic failures
in model precedence tests where a session override written to disk was
not observed by the next loadSessionStore() call.

Fix: add file size as a secondary cache invalidation signal. The cache
now checks both mtimeMs and sizeBytes — if either differs from the
cached values, it reloads from disk.

Changes:
- cache-utils.ts: add getFileSizeBytes() helper
- sessions/store.ts: extend SessionStoreCacheEntry with sizeBytes field,
  check size in cache-hit path, populate size on cache writes
- sessions.cache.test.ts: add regression test for same-mtime rewrite

src/config/sessions/store.ts

@@ -17,7 +17,12 @@ import {
   normalizeSessionDeliveryFields,
   type DeliveryContext,
 } from "../../utils/delivery-context.js";
-import { getFileMtimeMs, isCacheEnabled, resolveCacheTtlMs } from "../cache-utils.js";
+import {
+  getFileMtimeMs,
+  getFileSizeBytes,
+  isCacheEnabled,
+  resolveCacheTtlMs,
+} from "../cache-utils.js";
 import { loadConfig } from "../config.js";
 import type { SessionMaintenanceConfig, SessionMaintenanceMode } from "../types.base.js";
 import { enforceSessionDiskBudget, type SessionDiskBudgetSweepResult } from "./disk-budget.js";
@@ -39,6 +44,7 @@ type SessionStoreCacheEntry = {
   loadedAt: number;
   storePath: string;
   mtimeMs?: number;
+  sizeBytes?: number;
   serialized?: string;
 };
 
@@ -208,7 +214,8 @@ export function loadSessionStore(
     const cached = SESSION_STORE_CACHE.get(storePath);
     if (cached && isSessionStoreCacheValid(cached)) {
       const currentMtimeMs = getFileMtimeMs(storePath);
-      if (currentMtimeMs === cached.mtimeMs) {
+      const currentSizeBytes = getFileSizeBytes(storePath);
+      if (currentMtimeMs === cached.mtimeMs && currentSizeBytes === cached.sizeBytes) {
         // Return a deep copy to prevent external mutations affecting cache
         return structuredClone(cached.store);
       }
@@ -288,6 +295,7 @@ export function loadSessionStore(
       loadedAt: Date.now(),
       storePath,
       mtimeMs,
+      sizeBytes: getFileSizeBytes(storePath),
       serialized: serializedFromDisk,
     });
   }
@@ -667,6 +675,7 @@ function updateSessionStoreWriteCaches(params: {
     loadedAt: Date.now(),
     storePath: params.storePath,
     mtimeMs,
+    sizeBytes: getFileSizeBytes(params.storePath),
     serialized: params.serialized,
   });
 }