Expose fastMode field in custom status line JSON input

Problem

The custom status line command (statusLine.command in settings) receives a JSON blob via stdin with fields like model, effortLevel, context_window, rate_limits, etc. However, there is no field indicating whether fast mode is active.

This makes it impossible for custom status line scripts to show a fast mode indicator.

Requested Change

Add a fastMode boolean (or similar) to the JSON object passed to custom status line commands. Ideally this reflects the effective state (i.e., false if rate-limited even when toggled on), per the spirit of #27713.

Use Case

Users who customize their status line via statusLine.command want to display a fast mode indicator (e.g., ⚡️) next to the model name. Currently the only way to detect fast mode is through internal telemetry, which is not accessible from the status line script.

Context

• Related closed issue: #27713 (focused on the built-in indicator accuracy, not custom status line exposure)
• Fast mode is already tracked internally (fastMode: boolean in telemetry metadata) but not passed through to the status line JSON