[Feature Request] Expose initial context tokens separately in status line JSON metrics

Question

**Bug Description**
Feature request: Expose initial/static context size as a separate variable in the status line JSON data.
  Currently context_window metrics are aggregate — there's no way to distinguish initial context (system prompts,
  CLAUDE.md, MCP tool definitions) from conversation tokens. A field like context_window.initial_context_tokens would
  let users visualize how much context is consumed before they start chatting.

**Environment Info**
- Platform: win32
- Terminal: null
- Versi