Add CONTRIBUTING.md guide

CONTRIBUTING.md

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+# Contributing to the Databricks extension for VSCode
+
+Thank you for your interest in contributing! This document explains how to set
+up your environment, make a change, and open a pull request.
+
+## Found an issue?
+
+If you find a bug or have a feature request, please file an issue:
+https://github.com/databricks/databricks-vscode/issues/new
+
+When reporting a bug, please attach the extension logs as described here:
+https://docs.databricks.com/dev-tools/vscode-ext.html#send-usage-logs-to-databricks
+
+## Prerequisites
+
+- **Node.js** `>=22`
+- **Yarn** `3.2.x` (the repo pins `yarn@3.2.1` via `packageManager`)
+- **Python** (for the Python-side unit tests and the bundled runtime scripts)
+
+## Getting started
+
+Prepare Yarn and install dependencies:
+
+```sh
+npm install -g yarn@3
+yarn install
+```
+
+The Databricks JavaScript SDK (`@databricks/sdk-experimental`) is a regular npm
+dependency and is installed automatically by `yarn install` — no separate step
+is required.
+
+Fetch the Databricks CLI that the extension bundles:
+
+```sh
+yarn workspace databricks run package:cli:fetch
+```
+
+After that you're ready to build, run, and test the extension.
+
+## Building and running
+
+Build all workspaces:
+
+```sh
+yarn build
+```
+
+To develop the extension interactively, open the repository in VSCode and press
+`F5` to launch the Extension Development Host. Use the `watch` script for
+incremental rebuilds while you work:
+
+```sh
+yarn workspace databricks run watch
+```
+
+## Testing
+
+Run the full check (lint + unit tests) across all workspaces from the repo root:
+
+```sh
+yarn test
+```
+
+Within the `databricks-vscode` package you can run the individual suites:
+
+```sh
+# Lint (eslint + prettier check)
+yarn workspace databricks run test:lint
+
+# TypeScript unit tests (VSCode test runner)
+yarn workspace databricks run test:unit
+
+# Python unit tests
+yarn workspace databricks run test:python
+
+# Unit tests with coverage
+yarn workspace databricks run test:cov
+
+# End-to-end / integration tests (see "Integration test environment" below —
+# these require a live Databricks workspace and extra environment variables)
+yarn workspace databricks run test:integ
+```
+
+Please add tests for any new behavior, and make sure the full suite passes
+before opening a pull request.
+
+### Integration test environment
+
+Unlike the lint and unit suites, `test:integ` runs against a **live Databricks
+workspace**, so it needs credentials and a cluster before it will start. The
+e2e runner resolves authentication through the standard Databricks SDK unified
+auth (environment variables or a `.databrickscfg` profile), and it asserts that
+a default cluster is configured.
+
+Set the following before running `yarn workspace databricks run test:integ`:
+
+| Variable                                            | Required             | Description                                                                         |
+| --------------------------------------------------- | -------------------- | ----------------------------------------------------------------------------------- |
+| `DATABRICKS_HOST`                                   | Yes                  | Workspace URL, e.g. `https://my-workspace.cloud.databricks.com`.                    |
+| `TEST_DEFAULT_CLUSTER_ID`                           | Yes                  | ID of an existing cluster in that workspace; the runner starts it before the tests. |
+| `DATABRICKS_TOKEN`                                  | For PAT auth         | Personal access token. Provide this **or** the OAuth pair below.                    |
+| `DATABRICKS_CLIENT_ID` / `DATABRICKS_CLIENT_SECRET` | For OAuth (M2M) auth | Service principal credentials, used when no token is set.                           |
+
+For example, using a personal access token:
+
+```sh
+export DATABRICKS_HOST="https://my-workspace.cloud.databricks.com"
+export DATABRICKS_TOKEN="dapi..."
+export TEST_DEFAULT_CLUSTER_ID="0101-234567-abcdefgh"
+
+yarn workspace databricks run test:integ
+```
+
+Instead of exporting `DATABRICKS_HOST`/`DATABRICKS_TOKEN` you may point the SDK
+at an existing profile (for example `export DATABRICKS_CONFIG_PROFILE=DEFAULT`);
+`TEST_DEFAULT_CLUSTER_ID` is still required in that case. The extension e2e tests
+also build a `.vsix` package as part of `test:integ:prepare`, so make sure you
+have run `yarn build` first.
+
+## Code style
+
+Formatting and linting are enforced by prettier and eslint. Auto-fix
+formatting and lint issues with:
+
+```sh
+yarn fix
+```
+
+CI runs `yarn test`, which includes `test:lint`, so unformatted or lint-failing
+code will not pass.
+
+## Pull requests
+
+1. Fork the repository and create a topic branch from `main`.
+2. Keep each PR focused on a single change; split unrelated fixes or
+   refactors into separate PRs.
+3. Make sure `yarn test` passes and add tests for new behavior.
+4. Fill in the PR template — describe **what** changed and **how** it was
+   tested.
+5. Open your pull request. You do **not** need to sign anything up front — a
+   maintainer will request the Contributor License Agreement during review if
+   your change is accepted (see below).
+
+## Contributor License Agreement
+
+This project is licensed under the [Databricks License](LICENSE). To accept
+contributions, we need to confirm that you have the right to submit your
+contribution and that you grant Databricks the right to distribute it. For
+projects under the Databricks License this is done through a **Contributor
+License Agreement (CLA)** rather than a DCO commit sign-off.
+
+### How the CLA process works
+
+The CLA is handled manually by the maintainers — there is no self-service form
+to sign in advance:
+
+1. **Open your pull request** as normal. There is no need to sign anything
+   before you do — just author your commits with your real name and email (git
+   records these automatically).
+2. **A maintainer reviews your change.** If we would like to accept it, we will
+   send you the Databricks CLA and ask you, in a PR comment, to sign and return
+   it.
+3. **Sign the CLA** and return it as instructed, then confirm on the pull
+   request.
+4. **We merge** once the signed CLA is in place and the review is complete.
+
+Notes:
+
+- If you are contributing on behalf of a company, you may need internal
+  approval to sign the CLA, and it may require signature from an officer of the
+  company. Please start this process early so it doesn't block your PR.
+- Once the CLA is signed it covers subsequent contributions from the same
+  contributor or company — you only need to sign once.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the
+terms of the [LICENSE](LICENSE) in this repository (the Databricks License).