Add staged Copilot→Gemini migration playbook and wire it into contributor docs

[Copilot]

Summary

This PR implements the migration plan as repository documentation: a staged Copilot-to-Gemini transition guide with explicit execution checklists, decision gates, and an unresolved tool-mapping placeholder for “Google Antigravity.”

• Playbook added
  • Introduced docs/GEMINI_MIGRATION_PLAYBOOK.md with six migration phases:
    • inventory current Copilot usage
    • map use cases to Gemini tooling
    • run parallel adoption window
    • migrate prompts/guardrails
    • perform cutover with rollback ownership
    • stabilize with post-cutover metrics
• Operational structure included
  • Added concrete checklists, exit criteria, and a replacement matrix (Copilot use case → Gemini target).
  • Captured the open product-name ambiguity for “Google Antigravity” as an explicit follow-up action.
• Discoverability in main docs
  • Added README TOC entry and a dedicated section linking to the migration playbook.

## Copilot to Gemini Migration Playbook

If you are transitioning development workflows from GitHub Copilot to Google Gemini tooling, follow:

- [docs/GEMINI_MIGRATION_PLAYBOOK.md](docs/GEMINI_MIGRATION_PLAYBOOK.md)

Checklist

• I reviewed whether this change affects README, architecture docs, or legacy docs.
• If commands, supported file types, or layout metadata changed, I ran npm run docs:generate.
• I ran npm run docs:check.
• If historical Python prototype guidance changed, I updated docs/legacy-prototype.md.