UX: Consider clearing partial text on retry

[evansenter]

Summary

When an API call fails mid-stream and triggers a retry, any TextDelta events that were already emitted remain visible in the UI. This could cause brief "partial text" display followed by the full retried response.

Current Behavior

User: "Hello"
[partial response appears: "Hi there! I'm hap"]
[retrying in 2s: rate limited]
[full response appears: "Hi there! I'm happy to help..."]

The partial text and full response may both be visible.

Proposed Behavior

Options:

1. Track whether any text was emitted before retry, and emit a "clear" event for UI to handle
2. Document this as expected behavior (current approach - comment exists in code)
3. Buffer all TextDeltas until response completes, then emit (would lose streaming UX)

Related Code

src/agent.rs:386-388:

// If we had some response, clear it for the retry to avoid duplication
// (Note: TextDelta events were already sent, so UI might still show them)
full_response.clear();

Notes

This is a rare edge case that only occurs when:

1. The API fails mid-stream (after some text was streamed)
2. The error is retryable

Low priority as the current behavior is documented and the impact is minimal.