Use OSC 133 C and D to mark when prompt started and ended for terminal integration

### Describe the feature or problem you'd like to solve

Some terminals like Ghostty support notifications when a command completes that took more the X seconds, which is perfect for long prompt execution.

### Proposed solution

Write `\e]133;C\a` when a prompt is sent to the orchestrating (or only) agent and `\e]133;D\a` when it completes to have supporting terminals - including VSCode's integrated terminal, IIRC - notify the user that the agent has finished executing the prompt.

### Example prompts or workflows

I currently use some integration via oh-my-posh and Ghostty so that CLI commands that take more than 10 seconds notify me (system bell and/or notification window). To help support putting agent tasks in the background, OSC 133;C and D would notify me when the prompt in Copilot CLI (actually a TUI, so prompts don't work the same currently) is complete.

### Additional context

_No response_