feat(core): introduce MemoryProvider seam and MemoryService orchestrator

Refactors the existing background skill extractor into a layered memory
subsystem that GeminiClient can drive uniformly across interactive and
non-interactive sessions.

- Adds an internal `MemoryProvider` interface (not exported, not surfaced
  via extensions) as a typing seam between the orchestrator and its
  concrete implementation
- Refactors the existing skill-extraction logic into `DefaultMemoryProvider`,
  the sole implementation of the seam
- Introduces `MemoryService` as a thin orchestrator that owns a
  `DefaultMemoryProvider` and wraps every lifecycle call in try/catch so
  a buggy provider can't crash a turn
- Moves MemoryService ownership into `GeminiClient` (private field exposed
  via optional methods) so non-interactive entry points share the same
  lifecycle as the interactive UI
- Keeps builtin memory extraction asynchronous on startup to avoid
  blocking the first turn
- Adds unit tests for MemoryService, DefaultMemoryProvider, and the
  GeminiClient integration

The whole subsystem remains gated behind the existing
`isMemoryManagerEnabled()` experimental flag.

packages/cli/src/nonInteractiveCli.ts

@@ -529,6 +529,7 @@ export async function runNonInteractive(
       // Cleanup stdin cancellation before other cleanup
       cleanupStdinCancellation();
 
+      await config.getGeminiClient()?.shutdownSessionServices?.();
       scheduler?.dispose();
       consolePatcher.cleanup();
       coreEvents.off(CoreEvent.UserFeedback, handleUserFeedback);

packages/cli/src/nonInteractiveCliAgentSession.ts

@@ -616,6 +616,7 @@ export async function runNonInteractive({
       cleanupStdinCancellation();
       abortController.signal.removeEventListener('abort', abortSession);
 
+      await config.getGeminiClient()?.shutdownSessionServices?.();
       scheduler?.dispose();
       consolePatcher.cleanup();
       coreEvents.off(CoreEvent.UserFeedback, handleUserFeedback);

packages/cli/src/ui/AppContainer.tsx

@@ -92,7 +92,6 @@ import {
   ApiKeyUpdatedEvent,
   LegacyAgentProtocol,
   type InjectionSource,
-  startMemoryService,
 } from '@google/gemini-cli-core';
 import { validateAuthMethod } from '../config/auth.js';
 import process from 'node:process';
@@ -482,13 +481,6 @@ export const AppContainer = (props: AppContainerProps) => {
       setConfigInitialized(true);
       startupProfiler.flush(config);
 
-      // Fire-and-forget memory service (skill extraction from past sessions)
-      if (config.isMemoryManagerEnabled()) {
-        startMemoryService(config).catch((e) => {
-          debugLogger.error('Failed to start memory service:', e);
-        });
-      }
-
       const sessionStartSource = resumedSessionData
         ? SessionStartSource.Resume
         : SessionStartSource.Startup;
@@ -540,6 +532,7 @@ export const AppContainer = (props: AppContainerProps) => {
 
       // Fire SessionEnd hook on cleanup (only if hooks are enabled)
       await config?.getHookSystem()?.fireSessionEndEvent(SessionEndReason.Exit);
+      await config?.getGeminiClient()?.shutdownSessionServices?.();
     };
     registerCleanup(cleanupFn);