Add “Plan mode” to Copilot CLI for spec-first, step-by-step execution

[ans-4175]

Describe the feature or problem you'd like to solve

Copilot Chat VSCode can already plan and execute multi-step tasks, including editing files and running commands.

There is no explicit “plan-only” mode like Copilot Chat’s planning in VS Code, so users cannot easily review or persist a plan before execution.

Proposed solution

New command or mode:

copilot plan "Add bulk import command to our CLI"
copilot plan --file docs/bulk-import.spec.md "Add bulk import command to our CLI"
copilot implement --from-spec docs/bulk-import.spec.md

Plan mode outputs a markdown spec instead of executing actions, e.g.:
docs/<feature>.spec.md or .github/copilot/plan.<topic>.md.

Suggested minimal structure (compatible with spec-driven workflows):

# Plan

## Context

## Requirements

## Steps (checkbox todo list)

implement --from-spec reads the plan and applies Copilot CLI’s agentic capabilities to implement it, asking confirmation before running shell commands.

Integration & Options

Automatically include .github/copilot-instructions.md or similar project instructions into planning context.

Optional support for spec-kit-style layouts for teams already using SDD.

Benefits

Safer, reviewable automation in the terminal, mirroring Copilot Chat planning.
Specs live in the repo, enabling PR review and reuse by IDE agents (VS Code, etc.).

Example prompts or workflows

copilot plan "Add bulk import command to our CLI"
copilot plan --file docs/bulk-import.spec.md "Add bulk import command to our CLI"
copilot implement --from-spec docs/bulk-import.spec.md

Additional context

Plan mode outputs a markdown spec instead of executing actions