Add GitHub Actions: CI and a manual release workflow

- ci.yml: runs the unit test suite (npm test → node --test tests/unit) on
  push and PR to main, across ubuntu/windows/macos with Node 20. Electron is
  installed normally because the capture unit tests require app/capture.js,
  which require()s the electron module at load.

- release.yml: manual workflow_dispatch. Enter a version tag (e.g. v0.1.1);
  it builds the Windows portable .exe (npm run package:windows) and the Linux
  tarball + .deb (scripts/package-linux.sh), stamps the requested version onto
  each artifact, then publishes a GitHub Release at the current commit with
  the artifacts attached and auto-generated notes. Supports a prerelease flag.

.github/workflows/ci.yml

@@ -0,0 +1,37 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+# Cancel an in-progress run when newer commits are pushed to the same ref.
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: Unit tests (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      # The capture unit tests require app/capture.js, which require()s the
+      # electron module at load — so the Electron binary must actually be
+      # installed (don't set ELECTRON_SKIP_BINARY_DOWNLOAD here).
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run unit tests
+        run: npm test

.github/workflows/release.yml

@@ -0,0 +1,119 @@
+name: Release
+
+# Manual release: from the GitHub "Actions" tab pick "Release", click
+# "Run workflow", enter the version tag, and it builds the Windows portable
+# .exe and the Linux tarball + .deb, then publishes a GitHub Release with all
+# artifacts attached and auto-generated notes.
+on:
+  workflow_dispatch:
+    inputs:
+      version:
+        description: 'Release tag, e.g. v0.1.1 (the tag is created at the current commit on this branch)'
+        required: true
+        type: string
+      prerelease:
+        description: 'Mark this release as a pre-release'
+        required: false
+        default: false
+        type: boolean
+
+# Needed for the release job to create the tag and the GitHub Release.
+permissions:
+  contents: write
+
+jobs:
+  build-windows:
+    name: Build Windows portable
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      # Stamp the requested version onto package.json so the produced artifact
+      # carries it. Not committed back — it only affects this build.
+      - name: Set version from input
+        shell: bash
+        env:
+          VERSION: ${{ inputs.version }}
+        run: npm version "${VERSION#v}" --no-git-tag-version --allow-same-version
+
+      - name: Package Windows portable .exe
+        run: npm run package:windows
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: windows-portable
+          path: releases/*.exe
+          if-no-files-found: error
+
+  build-linux:
+    name: Build Linux tarball + .deb
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Set version from input
+        shell: bash
+        env:
+          VERSION: ${{ inputs.version }}
+        run: npm version "${VERSION#v}" --no-git-tag-version --allow-same-version
+
+      - name: Package Linux artifacts (tarball + .deb)
+        shell: bash
+        env:
+          STEPFORGE_PACKAGE_DIR: ${{ github.workspace }}/build/artifacts
+        run: bash scripts/package-linux.sh
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: linux-artifacts
+          path: build/artifacts/*
+          if-no-files-found: error
+
+  release:
+    name: Publish GitHub Release
+    needs: [build-windows, build-linux]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Download build artifacts
+        uses: actions/download-artifact@v4
+        with:
+          path: dist
+
+      - name: List downloaded artifacts
+        run: find dist -type f -printf '%s\t%p\n'
+
+      - name: Create GitHub Release
+        env:
+          GH_TOKEN: ${{ github.token }}
+          TAG: ${{ inputs.version }}
+          PRERELEASE: ${{ inputs.prerelease }}
+        run: |
+          flags=()
+          if [ "$PRERELEASE" = "true" ]; then
+            flags+=(--prerelease)
+          fi
+          gh release create "$TAG" \
+            dist/windows-portable/* \
+            dist/linux-artifacts/* \
+            --title "StepForge $TAG" \
+            --target "${{ github.sha }}" \
+            --generate-notes \
+            "${flags[@]}"