Add Changesets for per-package versioning and npm publish

- Install @changesets/cli and @changesets/changelog-github
- .changeset/config.json: public access, GitHub changelog, ignore private
  packages (monorepo root and gov-components)
- release.yml: replace tag-triggered publish with Changesets action —
  on every push to main it either opens a "Version Packages" PR (when
  changesets are pending) or publishes to npm (when that PR is merged)
- package.json: add changeset / version-packages / release convenience scripts

Contributor workflow:
  1. npx changeset   → pick packages + semver bump → commit the .md file
  2. Changesets bot opens Version PR bumping package.json versions
  3. Merge Version PR → CI publishes only the changed packages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sschupbach-salesforce

.changeset/README.md

@@ -0,0 +1,8 @@
+# Changesets
+
+Hello and welcome! This folder has been automatically generated by `@changesets/cli`, a build tool that works
+with multi-package repos, or single-package repos to help you version and publish your code. You can
+find the full documentation for it [in our repository](https://github.com/changesets/changesets).
+
+We have a quick list of common questions to get you started engaging with this project in
+[our documentation](https://github.com/changesets/changesets/blob/main/docs/common-questions.md).

.changeset/config.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://unpkg.com/@changesets/config@3.1.4/schema.json",
+  "changelog": ["@changesets/changelog-github", { "repo": "SFDC-Assets/gps-design-systems-react" }],
+  "commit": false,
+  "fixed": [],
+  "linked": [],
+  "access": "public",
+  "baseBranch": "main",
+  "updateInternalDependencies": "patch",
+  "ignore": ["@sfgps-ds/monorepo", "@gov-ido/gov-components"]
+}

.github/workflows/release.yml

@@ -2,19 +2,20 @@ name: Release
 
 on:
   push:
-    tags:
-      - 'v*'
+    branches: [main]
 
 jobs:
   release:
-    name: Publish to npm
+    name: Publish changed packages
     runs-on: ubuntu-latest
     permissions:
-      contents: read
+      contents: write
+      pull-requests: write
 
     steps:
       - uses: actions/checkout@9f698171ed81b15d1823a05fc7211befd50c8ae0 # v6.0.3
         with:
+          fetch-depth: 0
           token: ${{ secrets.GH_PAT != '' && secrets.GH_PAT || github.token }}
           persist-credentials: false
 
@@ -43,7 +44,13 @@ jobs:
       - name: Test (react-bridge)
         run: npm run test --workspace=packages/react-bridge
 
-      - name: Publish
-        run: npm publish --workspace=packages/react-uswds --access public
+      - name: Create Version PR or publish
+        id: changesets
+        uses: changesets/action@v1
+        with:
+          publish: npx changeset publish
+          title: 'chore: version packages'
+          commit: 'chore: version packages'
         env:
+          GITHUB_TOKEN: ${{ secrets.GH_PAT != '' && secrets.GH_PAT || github.token }}
           NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}