chore: stop publishing images from this repository (#134)

The Outlook add-in now builds and releases from encryption4all/postguard-js
(apps/outlook-addon), and production has been serving that build since
outlook-addin-v1.0.0. Until this workflow is removed, both repositories push to
the same GHCR package and can overwrite each other.

The collision is not theoretical. `build-edge` pushes
ghcr.io/encryption4all/postguard-outlook-addon:edge on every push to master,
and `edge` is exactly the tag the dev environment deploys
(postguard-ops environments/dev.tfvars). The monorepo now pushes the same tag
on every push to its main. A push here would therefore replace dev's image with
a build from this repository's source, which diverged from the monorepo copy at
import -- it is two majors behind on @e4a/pg-js, among other things.
`build-release` would do the same to `:latest`.

Removed rather than disabled with `if: false`: the monorepo carries the ported
equivalent, this repository is about to be archived, and the archive preserves
the file in history for anyone who wants to read it.

Neither remaining workflow can publish: ci.yml and pr-title.yml contain no
docker push, no ghcr.io reference, no `gh release create`, and no `packages:
write` or `contents: write` permission.

release-please-config.json and .release-please-manifest.json are left in place.
They are inert with no workflow reading them, and deleting them would only add
noise to a repository that is about to become read-only.

The README gains a pointer to where development moved, since GitHub's archive
banner says a repository is archived but not where it went.

Author: rubenhensen

.github/workflows/release.yml

@@ -2,6 +2,11 @@
 
 > For full documentation, visit [docs.postguard.eu](https://docs.postguard.eu/repos/postguard-outlook-addon).
 
+> [!IMPORTANT]
+> **Development moved to [encryption4all/postguard-js](https://github.com/encryption4all/postguard-js), under `apps/outlook-addon`.**
+> That repository builds and releases the add-in against the workspace copy of `@e4a/pg-js`; releases are tagged `outlook-addin-v*`.
+> This repository is kept for history and is about to be archived. It no longer publishes container images.
+
 Identity-based email encryption add-in for Microsoft Outlook. Users can send and receive encrypted email using [Yivi](https://yivi.app) identity verification, without needing to exchange keys. This is one of the main end-user clients for PostGuard, alongside the Thunderbird add-on.
 
 Targets the new Outlook on Windows and Outlook for Mac as a taskpane mail add-in (Compose + Read).