Feature request: expose token/context usage for external tools

Feature Request

Is your feature request related to a problem?

There's no way to display Claude Code's token/context usage in external monitoring tools (e.g., macOS menu bar apps like Stats, xbar/SwiftBar, or similar system monitors on other platforms).

Currently, token usage data is only visible inside the Claude Code TUI via /cost or the status line. Users who monitor system resources in their menu bar (CPU, RAM, disk, network) would love to also see Claude Code's context window usage at a glance.

Describe the solution you'd like

Expose session token/context usage in a machine-readable format, for example:

• Write usage stats to a file (e.g., ~/.claude/usage.json) updated in real-time, containing:
• context_used_percent (% of context window used)
• tokens_input / tokens_output
• cost_usd (session cost)
• model (current model)
• Alternatively, a local socket or CLI command (claude --usage --json) that returns current session stats

This would allow users to build integrations with menu bar tools (xbar, SwiftBar, Stats, Waybar, polybar, etc.) or custom dashboards.

Describe alternatives you've considered

• Relying on /cost inside the TUI — works, but requires switching context to the terminal
• Parsing terminal output — fragile and not officially supported

Additional context

Many developers already use menu bar monitors for CPU/RAM/network. Having Claude Code usage alongside those metrics would improve workflow ergonomics significantly.