Render markdown in submitted user messages in the timeline #5353

Describe the feature or problem you'd like to solve

Submitted user messages in the timeline/scrollback are shown as mostly raw text, so markdown in a prompt (lists, headings, code, emphasis) is not rendered the way the agent's own replies are.

Proposed solution

Render the user's submitted message with the same markdown renderer used for assistant output, inside the existing gray user-message block. For example, - item would show as a bulleted list, `code` as inline code, fenced blocks as code blocks, and **bold** as bold. Claude Code does this for user turns, and it makes long structured prompts much easier to scan when scrolling back through a session.

If some users prefer the literal text, a setting to keep raw display would cover that.

Example prompts or workflows

• Pasting a task spec with a heading and a bulleted list of requirements, then scrolling back to check what was asked.
• Prompts containing a fenced code block or stack trace, which is easier to read when shown as a code block.
• Multi-step instructions written as a numbered list.

Additional context

Copilot CLI 1.0.105, macOS. Currently a prompt like:

test

- example 1
- example 2

shows in scrollback with the first line styled bold and underlined, but - example 1 / - example 2 left as raw text rather than rendered as a list.

Related: #5084 (markdown not rendered in ask_user prompts), #3390 (user-message background block).