refactor: consolidate shared agent rules (#1786)

Move reusable path-scoped guidance out of GitHub Copilot configuration and into .agents/rules.\n\nAdd durable design principles, learning records, scoped guidance, deterministic validation, and non-mutating agent hooks while preserving clean-room gates.\n\nValidation: npm run check:agent-guidance; npm run test:agent-guidance; npm run test:agent-hooks; npm run lint:ci; npm test -- --run; npm run build; actionlint .github/workflows/agent-guidance.yml; git diff --check.