Rethink agent response rendering with custom backgrounds

When a custom background is enabled in the Agents window, the previously invisible structure of agent responses becomes prominent. The current bubble-less treatment uses mildly transparent response surfaces as a reader lane, but the separate containers, varying widths, and boundaries make the conversation feel fragmented and visually noisy.

The response rendering should be reconsidered so that it remains coherent with both the default background and custom backgrounds, without putting every agent response in a large chat bubble. User messages should remain distinct, while agent responses, questions, tool results, checkpoints, and status rows should read as one intentional hierarchy.

**Steps to reproduce**
1. Configure a custom background in the Agents window.
2. Open a chat containing user messages, agent questions, tool output, checkpoints, and completion/status rows.
3. Observe the translucent blocks and exposed response structure.

**Expected**
A calm, continuous reading lane whose hierarchy remains clear over custom backgrounds.

**Actual**
Agent responses appear as a collection of large, mismatched translucent cards, exposing implementation structure that is largely invisible with the default background.

_Created from a [Slack conversation](https://vscodeteam.slack.com/archives/C09MVCZKLP4/p1788263822598439?thread_ts=1788263822.598439&cid=C09MVCZKLP4)._