fix(storage): an unknown nested storage config can never silently land on the shared default root

The factory loudly rejects every REMOVED pre-8.0 path key, but an unknown
nested `config` object (e.g. `storage: { config: { baseDir } }` — a shape
that was never supported) fell through SILENTLY to the zero-config default
directory. Every instance constructed with such a shape wrote to ONE shared
on-disk root while its caller believed each had its own — found live when
two integration tests' brains shared a store across an entire
single-process CI run and a health probe refused on the foreign edges it
sampled. A nested `config` carrying any path-shaped key now throws the same
loud migration error, naming the canonical `path` rename. The two tests are
repaired to the supported shape (and now actually test isolated stores, for
the first time since 8.0).

src/storage/storageFactory.ts

@@ -154,6 +154,21 @@ export function resolveFilesystemRoot(
   ) {
     throwRemovedStorageKey('fileSystemStorage.path')
   }
+  // A nested `config` object carrying a path-shaped key is the same hazard in
+  // a shape nobody ever supported: it used to fall through SILENTLY to the
+  // shared default root — every instance writing one directory while its
+  // caller believed each had its own. (Found live: an integration test's
+  // brains shared one store across a whole single-process run and a health
+  // probe refused on the foreign edges it sampled.) Loud, with the rename.
+  const nested = (config as Record<string, unknown>).config
+  if (nested && typeof nested === 'object') {
+    const pathish = ['path', 'baseDir', 'rootDirectory', 'rootDir', 'dir', 'directory']
+    const hit = pathish.find(
+      (k) => typeof (nested as Record<string, unknown>)[k] === 'string' &&
+        ((nested as Record<string, unknown>)[k] as string).length > 0
+    )
+    if (hit) throwRemovedStorageKey(`config.${hit}`)
+  }
 
   // 3. Zero-config default. A `type: 'filesystem'` with no path lands here
   //    intentionally ("persist, default location").