Feature request: expand timeline by default (shell command output)

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

When Copilot CLI executes a shell command, the output is collapsed by default in the timeline (showing "2 lines..."). Users have to manually press Ctrl+O / Ctrl+E to expand all results. This creates friction when reviewing what commands actually did.

### Proposed solution

Add a user setting in settings.json (e.g. "expandTimelineByDefault": true) that automatically expands all shell command outputs in the conversation timeline.

This would benefit users who want full visibility into every command execution without needing to manually expand each result, improving transparency and trust in the agent's actions.

### Example prompts or workflows

1. User asks: "install the latest .NET 8 SDK" → Copilot runs `winget install`
→ output is collapsed, user cannot see if install succeeded without pressing Ctrl+O

2. User asks: "list installed .NET SDKs" → Copilot runs `dotnet --list-sdks`
→ result hidden by default, user misses the actual SDK versions returned

3. User asks: "what is my default .NET version?" → Copilot runs `dotnet --version`
→ answer is collapsed, user has to manually expand to see "10.0.202"

4. During a multi-step task (build, test, deploy), each command result is collapsed
→ user loses track of what succeeded or failed without manually expanding each step

### Additional context

The current workaround is Ctrl+O / Ctrl+E to expand all timeline entries manually, but this needs to be repeated every session. A persistent setting like "expandTimelineByDefault": true in ~/.copilot/settings.json would solve this cleanly without changing the default behavior for users who prefer the compact view.

Start by tracing the timeline handling for shell command output and the Ctrl+O/Ctrl+E expand-all behavior. Check how settings are loaded from ~/.copilot/settings.json, then add the proposed expandTimelineByDefault setting so shell output is expanded when enabled while the existing compact default remains unchanged.

索引モデルが issue の本文から書いたものです。