[FEATURE] Expose /usage and /context data in statusLine JSON

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

## Summary
Add subscription usage quota and context window metrics to the statusLine JSON input.

## Current Behavior
The statusLine script only receives:

• cost.total_cost_usd (API cost - not useful for subscription users)
• No context window free space percentage

## Requested Data

1. Usage/Quota info (like /usage command):

• Messages remaining
• Plan limits
• Reset time

1. Context window info (like /context command):

• Free space percentage
• Tokens used vs available

## Use Case
Subscription users want to monitor their remaining quota at a glance,
and all users benefit from seeing real context utilization.

Proposed Solution

Extend the statusLine JSON input with two new objects:

```json
{
"usage": {
"messages_remaining": 45,
"messages_limit": 50,
"reset_at": "2025-11-28T00:00:00Z",
"plan": "pro"
},
"context": {
"used_tokens": 78000,
"max_tokens": 200000,
"free_percent": 61.0
}
}

This data is already computed internally for /usage and /context commands,
so exposing it to statusLine should require minimal additional logic.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

_No response_

Additional Context

_No response_