Add release instructions

RELEASING.md

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+# Releasing
+
+This document describes how to create a new release of `linear-release`.
+
+## Prerequisites
+
+- You must be on the `main` branch with a clean working tree, up to date with `origin/main`
+- The [GitHub CLI](https://cli.github.com) (`gh`) must be installed and authenticated
+- `pnpm` must be installed
+
+## Creating a release
+
+Run the release script with the target version:
+
+```bash
+pnpm release <version>
+```
+
+For example:
+
+```bash
+pnpm release 0.7.0
+```
+
+The version must follow `MAJOR.MINOR.PATCH` format (e.g., `0.7.0`, `1.0.0`).
+
+## What happens
+
+The release script (`scripts/release.sh`) and CI workflows handle the full process:
+
+### 1. `pnpm release` (local)
+
+The script runs preflight checks and then:
+
+1. Validates that the version format is correct
+2. Checks that `gh` is installed and authenticated
+3. Verifies the working tree is clean, you're on `main`, and it's up to date with `origin/main`
+4. Ensures the `v<version>` tag and `release/<version>` branch don't already exist
+5. Creates a `release/<version>` branch
+6. Bumps the version in `package.json`
+7. Commits the change and pushes the branch
+8. Opens a PR against `main` via `gh pr create`
+
+### 2. PR review and merge
+
+Review and merge the PR as usual. The PR only contains the `package.json` version bump.
+
+### 3. Auto-tagging (CI)
+
+When a PR from a `release/*` branch is merged into `main`, the **Auto-tag release** workflow (`.github/workflows/auto-tag-release.yml`) runs automatically:
+
+1. Validates the version from the branch name matches `package.json`
+2. Creates and pushes a `v<version>` tag on the merge commit
+3. Triggers the release workflow
+
+### 4. Build and publish (CI)
+
+The **Release** workflow (`.github/workflows/release.yml`) is triggered by the new tag and:
+
+1. Builds platform-specific executables (linux-x64, darwin-x64, darwin-arm64) using Bun
+2. Code signs and notarizes the macOS binaries
+3. Creates a GitHub Release with the built binaries attached
+
+### 5. Update the GitHub action
+
+After publishing the new release, go to [linear-release-action](https://github.com/linear/linear-release-action) and update the [default pinned CLI version](https://github.com/linear/linear-release-action/blob/main/action.yml#L41) to the release you just created. When you've done so, publish a new release for the GitHub action as well.