[JITContext] Implement ContextManager and Feature Flag

[abhipatel12]

Description

Establish the core infrastructure for JIT context loading by introducing a feature flag and a new service to manage dynamic context state.

Acceptance Criteria

• [ ] A new boolean setting experimental.jitContext is available in Settings, defaulting to false.
• [ ] The Config class has an experimentalJitContext property initialized from settings.
• [ ] A new ContextManager class is created in packages/core/src/services/contextManager.ts.
• [ ] ContextManager can track loaded file paths to avoid duplicate loading.
• [ ] ContextManager implements a discoverContext(accessedPath: string) method that traverses upwards from the accessed path to the project root, finding relevant GEMINI.md files.
• [ ] The Config class initializes ContextManager only when the experiment flag is enabled.
• [ ] The Config class is updated to support separated memory tiers: globalMemory (Tier 1) and environmentMemory (Tier 2), while maintaining userMemory for legacy compatibility.

Technical Details

• Modify packages/cli/src/config/settingsSchema.ts: Add the new experimental setting jitContext.
• Create packages/core/src/services/contextManager.ts:
  • Implement ContextManager with a Set<string> for loadedPaths.
  • Implement discoverContext logic: upward traversal, file existence check, read file, process imports, return formatted string.
• Modify packages/core/src/config/config.ts:
  • Add experimentalJitContext to ConfigParameters and Config.
  • Add globalMemory and environmentMemory strings to Config.
  • Initialize this.contextManager in Config.initialize() if the flag is true.