Expose model-specific rate limits (e.g. Fable weekly) in statusline JSON

Feature request

The statusline JSON currently exposes two rate-limit windows under rate_limits:

"rate_limits": {
  "five_hour":  { "used_percentage": 30, "resets_at": 1738425600 },
  "seven_day":  { "used_percentage": 49, "resets_at": 1738857600 }
}

However, the /usage panel also shows model-specific rate limits — for example, "Current week (Fable)" with its own percentage and reset time. This data is not included in the statusline stdin JSON, so custom status line scripts cannot display it.

Proposal

Add model-specific rate-limit windows to the rate_limits object, e.g.:

"rate_limits": {
  "five_hour":    { "used_percentage": 30, "resets_at": 1738425600 },
  "seven_day":    { "used_percentage": 49, "resets_at": 1738857600 },
  "fable_weekly": { "used_percentage": 92, "resets_at": 1738857600 }
}

Or a more generic structure that scales to future per-model caps:

"rate_limits": {
  "five_hour": { ... },
  "seven_day": { ... },
  "model_specific": {
    "fable": { "used_percentage": 92, "resets_at": 1738857600 }
  }
}

Why

• The data already exists — /usage displays it.
• For users who rely heavily on Fable, hitting 90%+ on the Fable weekly cap without seeing it in the statusline is a surprise.
• The statusline is the right place for at-a-glance usage awareness; omitting a limit that can block requests defeats that purpose.

Screenshot

The /usage panel showing the Fable weekly limit that is invisible to statusline scripts:

<img width="1370" height="1960" alt="Image" src="https://github.com/user-attachments/assets/018f8510-2c52-4c45-9fba-720453bf3b06" />