ci: announce releases on Slack from the release workflow (#75)

* ci: announce releases on Slack from the release workflow

When changesets/action reports a package was published, resolve the
release URL from publishedPackages and post to the Slack Workflow
Builder trigger via slackapi/slack-github-action (webhook-trigger
mode). Mirrors the pattern in slackapi/node-slack-sdk's release.yml.

Both new steps gate on steps.changesets.outputs.published == 'true',
so nothing fires on the "opens/updates the release PR" runs — only on
the run that follows merging that PR.

Also updates the maintainers guide so step 5 reflects that the Slack
announcement is now automated.

* chore: clarify changeset audience is the internal maintainers channel

* ci: simplify publication-details step to match bolt-js pattern

Replace the jq-templated tag URL with `gh release view --json url`, and
switch the message shape to `jq | paste -sd ', ' -`. Same behavior for
a single-package release (`slack@<version>`), less indirection, and
matches slackapi/bolt-js/release.yml:107-108.

Also renames the URL output from `action_url` to `url` to match the
same reference, and threads `GH_TOKEN` into the step's env so `gh`
can auth.

Author: mwbrooks

.changeset/announce-releases-on-slack.md

@@ -0,0 +1,5 @@
+---
+"slack": patch
+---
+
+Automate release announcements to the internal maintainers' Slack channel from the release workflow.

.github/maintainers_guide.md

@@ -88,8 +88,7 @@ New official package versions are published when the release PR created from cha
 
 4. **Merge and approve**: Merge the release PR. It may take up to 24 hours before you see you release in the [Claude Plugins](https://claude.com/plugins/slack) directory.
 
-5. **Communicate the release**:
-   - **External**: Post in relevant channels (e.g. #lang-javascript, #tools-bolt) on [Slack Community](https://community.slack.com/). Include a link to the release notes.
+5. **Communicate the release**: A Slack announcement is posted automatically to the release-announcements channel by `.github/workflows/release.yml` when the release PR is merged and a tag is cut. For broader outreach (e.g. `#tools-bolt` on [Slack Community](https://community.slack.com/)), post manually if desired.
 
 ## Everything Else
 

.github/workflows/release.yml

@@ -37,6 +37,7 @@ jobs:
       - name: Install Node dependencies
         run: npm install
       - name: Changesets release
+        id: changesets
         uses: changesets/action@a45c4d594aa4e2c509dc14a9f2b3b67ba3780d0d # v1.9.0
         with:
           version: bash scripts/changeset_version.sh
@@ -47,3 +48,22 @@ jobs:
           prDraft: create
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Get publication details
+        id: release
+        if: steps.changesets.outputs.published == 'true'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PUBLISHED_PACKAGES: ${{ steps.changesets.outputs.publishedPackages }}
+        run: |
+          echo "url=$(gh release view --json url -q .url)" >> "$GITHUB_OUTPUT"
+          echo "message=$(echo "$PUBLISHED_PACKAGES" | jq -r '.[] | "\(.name)@\(.version)"' | paste -sd ', ' -)" >> "$GITHUB_OUTPUT"
+      - name: Notify Slack
+        if: steps.changesets.outputs.published == 'true'
+        uses: slackapi/slack-github-action@45a88b9581bfab2566dc881e2cd66d334e621e2c # v3.0.3
+        with:
+          webhook: ${{ secrets.SLACK_RELEASE_ANNOUNCEMENTS_WEBHOOK_URL }}
+          webhook-type: webhook-trigger
+          payload: |
+            action_url: "${{ steps.release.outputs.url }}"
+            message: "${{ steps.release.outputs.message }}"
+            repository: "${{ github.repository }}"