[FEATURE] VS Code extension: make pinned initial prompt collapsible

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The initial user message is pinned at the top of the chat panel. When the first message is long (detailed task description, prior work, multi-step plans), the overlay consumes the entire viewport and hides all responses.

Proposed Solution

Suggestion: Add a collapse/expand chevron so users can click to shrink it to a single summary line. Alternatively, a setting to disable pinning, a max-height with scroll, or auto-collapse after first response.

Alternative Solutions

_No response_

Priority

Critical - Blocking my work

Feature Category

Interactive mode (TUI)

Use Case Example

1. I start a task by pasting a detailed prompt (~30 lines) with task description, prior work, architectural decisions, and stakeholder info
2. The initial message gets pinned at the top of the chat panel as an overlay
3. The pinned overlay consumes the entire viewport — I can't see any of Claude's responses
4. I have to full-screen the panel or scroll past the pinned area to read anything
5. With a collapse chevron, I could click once to shrink it to a single line and see the full conversation normally
6. This would save time because I frequently start sessions with structured task briefs and shouldn't have to fight the UI to read responses

Additional Context

_No response_