Feature: /context --brief flag (chart only, no listings)

Summary

Add a --brief flag to the built-in /context command that shows only the utilization chart and categorical breakdown, without the MCP tools, custom agents, memory files, and skills listings below it.

Motivation

The /context output has two distinct sections:

1. The utilization chart — the colored bar with categorical breakdown (system prompt, tools, agents, memory, skills, messages, free space) and token counts
2. The inventory listings — full lists of MCP tools, custom agents, memory files, and skills with individual token counts

Section 1 is what users check most frequently ("how much runway do I have and what's consuming it?"). Section 2 is useful for auditing but rarely needed on a quick glance.

Currently there's no way to get just the chart. A --brief flag would make quick context checks fast and low-noise.

Alternative considered

Building this as a custom slash command using the statusline bridge file (/tmp/claude-ctx-{session_id}.json), but that file only contains aggregate used_pct / remaining_percentage — no categorical breakdown. The categorical data is computed inside the CLI when assembling the prompt and isn't exposed to hooks or external tools.

Bonus: expose categories in statusline hook data

If the categorical breakdown were included in the statusline hook's input data (alongside context_window.remaining_percentage), the community could build custom visualizations without needing CLI changes. Something like:

{
  "context_window": {
    "remaining_percentage": 94.3,
    "categories": {
      "system_prompt": 7000,
      "system_tools": 10300,
      "custom_agents": 3300,
      "memory": 887,
      "skills": 2400,
      "messages": 60
    }
  }
}

This would be the more extensible solution long-term.