Add Copilot onboarding instructions for ACK monorepo

[Copilot]

This PR onboards btwld/ack for first-time Copilot coding agents by adding repository-specific working guidance in .github/copilot-instructions.md. It captures the project’s execution model and explicitly documents onboarding/CI friction points with practical workarounds.

• What was added

  • New file: .github/copilot-instructions.md
  • Agent-focused guidance for:
    • monorepo/package layout (ack, ack_annotations, ack_generator, adapters, example)
    • setup prerequisites and bootstrap flow (Dart/Flutter, Melos, optional setup.sh)
    • canonical commands for analysis/tests/build/golden/json-schema validation
    • change-scope expectations (minimal, package-scoped, avoid manual edits to generated files)

• CI and release context captured

  • References the delegated CI workflow model (.github/workflows/ci.yml → shared dart-actions workflow with DCM)
  • Notes Conventional Commit expectation and publishing/versioning entrypoints from PUBLISHING.md

• Documented errors + workarounds (per issue requirement)

  • melos: command not found → install/activate Melos or use dart run melos ...
  • dart: command not found → install Dart SDK (or run ./setup.sh) before bootstrap
  • CI run in action_required with no jobs → maintainer approval/permissions required before rerun

## Commands you should run
- Full CI-equivalent local check: `melos run test --no-select`
  - Runs strict analyze (`dart analyze . --fatal-infos`) and package tests.

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