Add toggle to hide 'context left until auto-compact' warning

Feature Request

Add a configuration option to hide or disable the "context left until auto-compact" warning message that appears in the UI.

Use Case

During long interactive sessions (e.g., visual graph exploration with frequent screenshots and tool calls), the auto-compact warning becomes distracting. Users who understand context compaction and are comfortable with it don't need the persistent reminder.

Proposed Solution

A toggle in settings or .claude/settings.json such as:

{
  "ui": {
    "showCompactionWarning": false
  }
}

Or a CLI flag: claude --hide-compact-warning

Current Behavior

The warning appears automatically as context fills up. There is no way to dismiss or disable it.

Desired Behavior

Users can opt out of the warning via settings, while keeping it enabled by default for new users who may not understand context compaction.