feat: Add release-prep notifications

[Dumea Alexandru (alexdumea)]

Add version-bump action + version-bump-notify reusable workflow

Why:
We had no signal for when someone is preparing to release a plugin.
This adds an early "a release is being prepared" alert to Slack, the moment that version is bumped

What it does:

• Detects a strict increase of the root version in composer.json between two git refs.
• On a real bump, posts a Slack message (plugin name, old → new, author, link to the PR/commit).
• Stays silent on everything that isn't a real bump: no-op edits, downgrades, reverts, and v1.0.0 → 1.0.0-style no-ops.

[Copilot]

Pull request overview

Adds a new “version-bump” composite action plus a reusable Slack-notification workflow to signal when an extension’s composer.json version has been increased (i.e., release preparation), and wires basic CI coverage for the new logic.

Changes:

• Introduces version-bump composite action to detect strict version increases between two git refs.
• Adds reusable workflow version-bump-notify.yml to post a Slack alert when a version bump is detected.
• Adds bash-based tests and documentation, and registers the new action in repo CI and top-level README.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
version-bump/version-bump.bash	Implements version bump detection by reading composer.json at two refs and comparing versions.
version-bump/version-bump.test.bash	Adds a bash test harness with a stubbed git show to validate bump/no-bump scenarios.
version-bump/action.yml	Defines the composite action interface (inputs/outputs) and runs the detection script.
version-bump/README.md	Documents action behavior, inputs/outputs, and usage example.
.github/workflows/version-bump-notify.yml	Adds reusable workflow that calls the action and posts a Slack message when bumped.
.github/workflows/test.yml	Extends CI workflow to run the new version-bump tests.
README.md	Lists the new notification-related action in the repository catalog.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

[Dominik Grothaus (dgrothaus-sw)]

The whole logic looks solid. But I would recommend going another route by not simply triggering on change like it is implemented now.

Now it notifies like this:

1. PR is opened, contains compose.json, notification sent
2. PR is opened, version bump forgotten, add commit with version bump, no notification sent.

I commented two little changes to make this notification a bit more useful for the 2. case without spamming on every commit when synchronize is watched.