feat(core): Implement JIT context memory loading and UI sync

[SandyTao520]

TLDR

Refactors memory loading to support the Experimental JIT Context feature. It introduces a unified refresh() method in ContextManager for JIT mode, ensures the UI receives updates via events, and properly includes environment memory (and MCP instructions) in the system context.

Dive Deeper

• ContextManager Refactor: Introduced a refresh() method to unify memory reloading (Global + Environment) and event emission. Removed legacy loadGlobalMemory calls from Config initialization.
• Config Delegation: Config delegates memory state retrieval (file count, paths) to ContextManager when JIT is enabled.
• Event Optimization: Simplified MemoryChanged event to only emit fileCount, as content and paths are pulled from Config on demand or not required for simple UI updates.
• Memory Refresh: Updated /memory refresh command to use ContextManager.refresh() when JIT is enabled.
• System Context: getEnvironmentContext now includes the content of loaded environment memory files. MCP instructions are appended to environment memory in JIT mode.

Reviewer Test Plan

1. Enable JIT: Set experimental.jitContext: true in settings.
2. Startup: Launch the CLI. Verify that the "Using: X GEMINI.md files" indicator appears and shows the correct count.
3. Context: Run /reset and start a chat. Verify (via debug logs or model response) that the system instruction includes content from your GEMINI.md files AND any active MCP server instructions.
4. Refresh: Modify a GEMINI.md file and run /memory refresh. Verify the count updates and the model sees the new content.
5. Legacy: Disable JIT. Verify that memory loading works as before (using the CLI-side loader).

Testing Matrix

	🍏	🪟	🐧
npm run	✅	❓	❓
npx	✅	❓	❓
Docker	❓	❓	❓
Podman	❓	-	-
Seatbelt	❓	-	-

Linked issues / bugs