Adds copilot agent guidelines (#3048)

* Adds copilot agent guidelines

* Update docs/pr-guidelines/pull-request-guidance.md

Co-authored-by: Copilot <[email protected]>

* Update docs/pr-guidelines/pull-request-guidance.md

Co-authored-by: Copilot <[email protected]>

* Update .github/copilot-instructions.md

Co-authored-by: Copilot <[email protected]>

---------

Co-authored-by: Copilot <[email protected]>

Author: varadarajan-tw

.github/copilot-instructions.md

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+# Copilot Instructions
+
+This document provides instructions for GitHub Copilot to assist with reviewing pull requests and performing tasks based on user prompts. It references existing guidelines and role-specific instructions available in this repository.
+
+## Understanding the repository
+
+Read the [README](../README.md) to understand the purpose of this repository and its structure. On high level, this repository contains the following key packages:
+
+- [`action-destinations`](../packages/destination-actions): This contains all cloud-mode destinations. All new destinations are registered manually and the destination metadata id is then updated in [index.ts](../packages/destination-actions/src/index.ts).
+- [`browser-destinations`](../packages/browser-destinations): This contains all browser-mode destinations. All new destinations are registered manually and the destination metadata id is then updated in [destination-manifest/index.ts](../packages/destinations-manifest/src/index.ts).
+- [`core`](../packages/core): This contains all the core framework for running the destinations. Any changes made here will affect both cloud-mode and browser-mode destinations. Thorough regression testing is required for these changes.
+- [`destination-subscriptions`](../packages/destination-subscriptions): This package validates event payloads against an action's subscription AST.
+- [`browser-destination-runtime`](../packages/browser-destination-runtime): This package contains the runtime for browser-mode destinations, including the logic for executing actions and handling events.
+
+## Reviewing Pull Requests
+
+When reviewing pull requests, Copilot should:
+
+1. Ensure all the required checks mentioned [here](../docs/pr-guidelines/pr-checks.md) have passed.
+2. Follow the expectations outlined in the [Reviewer Guidelines](../docs/pr-guidelines/pull-request-guidance.md).
+3. Suggest splitting changes to multiple destinations into separate pull requests.
+4. Ensure that the pull request doesn't introduce known breaking changes as called out in the [Pull Request Guidance](../docs/pr-guidelines/pull-request-guidance.md).
+
+## Performing Tasks
+
+When performing tasks based on user prompts, Copilot should:
+
+1. Refer to the relevant guidelines and documentation in this repository.
+2. Ensure that any code changes follow the principles outlined in the [Contributor Guidelines](../docs/pr-guidelines/pull-request-guidance.md).
+3. Ensure that code changes are implemented using as many framework features as possible, such as conditionally required fields, error handling etc.
+4. Write clear unit tests for any new functionality or changes made to existing functionality.
+   - Use the existing tests as a reference for writing new tests.
+   - All external calls should be mocked in the tests. For cloud-mode destinations, we use `nock` to mock external calls.

docs/pr-guidelines/pr-checks.md

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+# PR CI Checks
+
+The following checks should pass for all PRs:
+
+- CI / Unit Tests
+- CI / Lint
+- CI / Validate
+- CI / Browser Destination Bundle QA
+- CI / Browser tests: actions-core
+- Required Field Check / required-field-check
+- Test External
+- codecov

docs/pr-guidelines/pull-request-guidance.md

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+# PR request guidelines
+
+- The pull request includes a description of testing performed.
+- The pull request doesn't introduce a new required field to an already existing action definition. This will result in failures for existing customers.
+- Ensure errors are handled as per [error-guidelines](../docs/error-handling.md).
+- Ensure the pull request uses framework features such as [Conditionally required fields](https://github.com/segmentio/action-destinations?tab=readme-ov-file#required-fields)
+- Check for grammatical and spelling mistakes in descriptions for Action Definition fields and Destination Definition settings.