Add Copilot CLI support as a first-class Ralph agent

[acypert]

Description

This PR adds full support for GitHub Copilot CLI in Ralph so it can be selected and used like existing agents
(codex/claude/droid/opencode), including install-time setup and runtime command resolution.

What changed

1. Added copilot agent command defaults and interactive command wiring in .agents/ralph/agents.sh,
   .agents/ralph/loop.sh, and bin/ralph.
2. Extended CLI options to include:

• --agent ...|copilot
• --copilot-reasoning-effort <low|medium|high>

3. Updated ralph install --skills flow to support copilot, including prompts for model and reasoning effort, and
   auto-writing AGENT_CMD/PRD_AGENT_CMD into .agents/ralph/config.sh.
4. Added Copilot install hint (npm install -g @github/copilot) where missing-agent guidance is shown.
5. Updated docs and examples (README.md, examples/commands.md) to include Copilot usage and the new reasoning-effort
   option.
6. Updated tests to cover Copilot in ping/loop paths and refreshed audit notes (tests/agent-ping.mjs,
   tests/agent-loops.mjs, tests/audit.md).

Why

This enables teams using Copilot CLI to run Ralph workflows without custom local patching, and keeps Copilot
configuration (model/effort) consistent across project setup and execution.