[JITContext] Refactor Memory Discovery for JIT Loading

Description

Modify the existing memory discovery logic to support a non-recursive mode that only loads Tier 1 (Global) and Tier 2 (Project Root + Extensions) memory when the feature flag is enabled.

Acceptance Criteria

• [ ] loadServerHierarchicalMemory accepts an experimentalJitContext flag.
• [ ] When the flag is true, loadServerHierarchicalMemory skips the recursive bfsFileSearch.
• [ ] When the flag is true, it still correctly finds and loads the Global GEMINI.md and the Project Root GEMINI.md.
• [ ] loadServerHierarchicalMemory returns a structured result containing globalMemory, environmentMemory, and initial loadedPaths.
• [ ] The CLI's loadCliConfig correctly passes the feature flag and handles the structured response to populate the Config object.
• [ ] Legacy behavior (recursive search) is preserved when the flag is false.

Technical Details

• Modify packages/core/src/utils/memoryDiscovery.ts:
  • Update loadServerHierarchicalMemory signature and return type.
  • In getGeminiMdFilePathsInternalForEachDir, conditionally bypass bfsFileSearch based on the flag.
• Modify packages/cli/src/config/config.ts:
  • Update loadHierarchicalGeminiMemory to pass the flag.
  • Update loadCliConfig to read the flag from settings and use the new structured response from loadHierarchicalGeminiMemory.