fix(jsonl): reject unusable roots at plugin load

A configured root that already exists as a file or unreadable directory cannot host cwd buckets, but the backend previously mounted and deferred that deterministic configuration error until a later list or write.

Probe the resolved root while the plugin loads, surface every error except ENOENT, and keep an absent root valid for lazy first materialization. Document the timing contract, regenerate the config catalog, and pin the non-directory case at the load boundary.

packages/session-persistence/session-persistence-jsonl/tests/jsonl.spec.ts

@@ -748,15 +748,12 @@ describe('SessionPersistenceJsonl: edge cases', () => {
     await ctx2.fiber.dispose()
   })
 
-  it('list surfaces a non-ENOENT root error (ENOTDIR) instead of reporting no sessions', async () => {
-    // A durable backend must not collapse a storage fault to "no sessions". Making the root a
-    // regular file forces ENOTDIR from `readdir`, which must propagate.
+  it('plugin load rejects an existing root that is not a directory', async () => {
     const filePath = join(root, 'not-a-dir')
     await writeFile(filePath, 'x')
     const ctx2 = new Context()
     await ctx2.plugin(SessionStore)
-    await ctx2.plugin(SessionPersistenceJsonl, { root: filePath })
-    await expect(ctx2.sessionPersistence.list()).rejects.toThrow(/ENOTDIR/)
+    await expect(ctx2.plugin(SessionPersistenceJsonl, { root: filePath })).rejects.toThrow(/ENOTDIR/)
     await ctx2.fiber.dispose()
   })