docs(agent): remove stale BuiltinMemoryProvider references from memory module docstrings

The BuiltinMemoryProvider class was removed from the codebase but its
name lingered in the module-level docstrings of memory_manager.py and
memory_provider.py, creating false expectations:

- memory_manager.py docstring showed example code doing
  add_provider(BuiltinMemoryProvider(...)) which ImportError at runtime
- memory_provider.py docstring listed BuiltinMemoryProvider as
  'always present, not removable' — misleading for new contributors

The regression test (test_memory_user_id.py) already passes without
any reference to BuiltinMemoryProvider; it uses RecordingProvider
instances directly. The stale references were docs-only drift.

Update both docstrings to reflect the actual current architecture:
MemoryManager accepts external plugin providers only (one at a time).

Closes #14402

agent/memory_manager.py

@@ -1,17 +1,14 @@
-"""MemoryManager — orchestrates the built-in memory provider plus at most
-ONE external plugin memory provider.
+"""MemoryManager — orchestrates memory providers for the agent.
 
 Single integration point in run_agent.py. Replaces scattered per-backend
 code with one manager that delegates to registered providers.
 
-The BuiltinMemoryProvider is always registered first and cannot be removed.
-Only ONE external (non-builtin) provider is allowed at a time — attempting
-to register a second external provider is rejected with a warning.  This
+Only ONE external plugin provider is allowed at a time — attempting to
+register a second external provider is rejected with a warning.  This
 prevents tool schema bloat and conflicting memory backends.
 
 Usage in run_agent.py:
     self._memory_manager = MemoryManager()
-    self._memory_manager.add_provider(BuiltinMemoryProvider(...))
     # Only ONE of these:
     self._memory_manager.add_provider(plugin_provider)