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
+"""MemoryManager — orchestrates memory providers for the agent.
-ONE external plugin memory provider.
 
 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 plugin provider is allowed at a time — attempting to
-Only ONE external (non-builtin) provider is allowed at a time — attempting
+register a second external provider is rejected with a warning.  This
-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)
 

agent/memory_provider.py

@@ -1,17 +1,16 @@
 """Abstract base class for pluggable memory providers.
 
-Memory providers give the agent persistent recall across sessions. One
+Memory providers give the agent persistent recall across sessions.
-external provider is active at a time alongside the always-on built-in
+The MemoryManager enforces a one-external-provider limit to prevent
-memory (MEMORY.md / USER.md). The MemoryManager enforces this limit.
+tool schema bloat and conflicting memory backends.
 
-Built-in memory is always active as the first provider and cannot be removed.
+External providers (Honcho, Hindsight, Mem0, etc.) are registered
-External providers (Honcho, Hindsight, Mem0, etc.) are additive — they never
+and managed via MemoryManager. Only one external provider runs at a
-disable the built-in store. Only one external provider runs at a time to
+time.
-prevent tool schema bloat and conflicting memory backends.
 
 Registration:
-  1. Built-in: BuiltinMemoryProvider — always present, not removable.
+  Plugins ship in plugins/memory/<name>/ and are activated via
-  2. Plugins: Ship in plugins/memory/<name>/, activated by memory.provider config.
+  the memory.provider config key.
 
 Lifecycle (called by MemoryManager, wired in run_agent.py):
   initialize()          — connect, create resources, warm up