Persistent Statusline bar (inspired by Claude Code) #2266

Describe the feature or problem you'd like to solve

No response

Proposed solution

Claude Code ships a statusline today (https://code.claude.com/docs/en/statusline) — a persistent bottom bar that runs a user-defined shell script.

Proposed design for Copilot CLI:

• Config: statusLine field in ~/.copilot/config.json pointing to a shell script or inline command
• Data: Script receives JSON session data on stdin (model name, context %, cwd, git branch, session cost)
• Display: Persistent bottom bar showing whatever the script prints (supports ANSI colors, multi-line)
• Command: /statusline to generate scripts from natural language
• Updates: Refresh after each assistant message (debounced ~300ms)

Example prompts or workflows

1. /statusline show model name and context percentage with a progress bar
2. /statusline show git branch, modified file count, and session cost
3. A statusline.sh that color-codes context usage: green <50%, yellow 50-80%, red >80%
4. Multi-line bar: git info on line 1, context + cost on line 2
5. Inline jq command: jq -r '"[\(.model.display_name)] \(.context_window.used_percentage)% context"'

Additional context

This would help avoid surprise context compactions, distinguish multiple sessions at a glance, and keep git branch always visible when switching repos.