Status line should refresh periodically during long turns

Problem

The status line command only executes between turns. During a single long turn (e.g., an agent making many file edits over several minutes), the status line data becomes stale.

For example, if the status line displays git diff stats (+N -N), these numbers won't update until the entire turn completes — even though dozens of files may have been modified mid-turn. This can be confusing, as users may think the stats are broken or reflecting the wrong branch.

Proposed Solution

Refresh the status line command periodically during a turn, such as:

• After each tool call completes, or
• On a timer (e.g., every 10-30 seconds while a turn is active)

Context

Status line is configured via settings.json:

{
  "statusLine": {
    "type": "command",
    "command": "bash /path/to/statusline-command.sh"
  }
}

The command itself is lightweight (a few git calls + jq), so frequent execution should not cause performance issues.