Add GitHub Copilot CLI guidance to readme

[netgio]

Description

Added mention of GitHub Copilot CLI for agent selection based on docs at https://docs.github.com/en/copilot/how-tos/copilot-cli/use-copilot-cli/invoke-custom-agents#use-custom-agents

Help new GitHub Copilot CLI users quickly invoke spec-kit agent commands.

Testing

• Tested locally with uv run specify --help
• [ x] Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

AI Disclosure

• [x ] I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates the README guidance for launching a coding agent by adding a note about selecting/targeting the agent with GitHub Copilot CLI.

Changes:

• Extend the “Establish project principles” section with an additional clause describing GitHub Copilot CLI agent selection.

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