[FEATURE] Expose context breakdown by component in statusline API

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The statusline API currently provides total token usage but doesn't break down context consumption by component. Adding this breakdown would enable real-time monitoring of context efficiency.

Proposed Solution

Expose the same component breakdown that /context command shows:

{
  "context_window": {
    "context_window_size": 200000,
    "current_usage": {
      "input_tokens": 50000,
      "cache_creation_input_tokens": 10000,
      "cache_read_input_tokens": 5000
    },
    "breakdown": {
      "system_prompt": 5000,
      "system_tools": 8000,
      "mcp_tools": 12000,
      "memory_files": 3000,
      "custom_agents": 2000,
      "messages": 20000
    }
  }
}

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

This would enable statusline scripts to:
Visualize context composition - Show how much space MCP tools, system prompt, and messages consume
Alert on bloat - Warn when MCP tools exceed a threshold (e.g., >25% of context)
Optimize dynamically - Help users decide when to disable MCP servers based on real-time data
Track message growth - Show conversation tokens separately from static overhead

[■■■■■■■■■■■■■■■■■■■■■■■■■■■■|□□□□□□□□□□□□] 70%
sys:5k | tools:8k | mcp:12k | msgs:115k | 60k free

Additional Context

I found few issues related but not getting this high level view imo.