Add subscription usage data to statusline context

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 hook currently receives context data (model info, context_window usage, cost data) but does NOT include subscription usage information. Users with Claude Max/Pro subscriptions cannot monitor their usage percentage or reset time in their custom statusline without manually running the /usage command.

Proposed Solution

Add subscription usage fields to the statusline context JSON that is passed to custom statusline scripts:

1. subscription_usage.percentage - Current usage percentage of subscription limit (0-100)
2. subscription_usage.reset_timestamp - ISO timestamp when the usage resets
3. subscription_usage.plan_tier - Plan name (e.g., "Max", "Pro")
4. subscription_usage.messages_used - Number of messages used (optional)
5. subscription_usage.messages_limit - Total message limit (optional)

Alternative Solutions

_No response_

Priority

Low - Nice to have

Feature Category

Developer tools/SDK

Use Case Example

I have a custom statusline script (~/.claude/statusline-command.sh) that displays model name, git branch, and context usage. I want to also display my subscription usage like:
[Opus] main | 42% subscription | resets in 3d 5h

This would let me monitor my Claude Max usage at a glance without interrupting my workflow to run /usage manually.

Additional Context

The infrastructure already exists - the statusline hook receives rich JSON context including cost and context_window data. Adding subscription data would be a natural extension. This is particularly valuable for Max/Pro subscribers who have monthly message limits and want to pace their usage throughout the billing period.