Start in Plan mode by default

[RockNoggin]

Describe the feature or problem you'd like to solve

There's no way to default Copilot CLI to plan mode. Users must manually switch via Shift+Tab or /plan each session, which is
easy to forget — leading to unintended code edits before a plan can be reviewed.

Proposed solution

Add a --plan CLI flag and/or a persistent config option (e.g., "defaultMode": "plan") so sessions start in plan mode
automatically.

Example prompts or workflows

1. copilot --plan — Launch Copilot CLI directly in plan mode via a command-line flag.
2. A persistent config option (e.g., in ~/.copilot/config.json) like "defaultMode": "plan" so every new session starts in plan mode without needing the flag.
3. Typical workflow: I run copilot, describe a feature idea, and expect to review a structured plan before any code changes happen. Without defaulting to plan mode, the agent immediately begins editing files, which requires me to undo unwanted changes.

Additional context

I often forget to enable plan mode when I'm excited about an idea, Copilot then starts making code edits while I was expecting the plan to approve.