ci(mkosi): title guest OS releases after the product, not the backend - #1142

Problem

The mkosi release job publishes with --title "$TAG", and the tag is mkosi-os-v<version>. That prefix exists only so this workflow's tag trigger cannot collide with the Yocto guest-OS one — it is a CI implementation detail, not a product name. It nevertheless ends up as the release name users read on the releases page, next to entries like Verifier Release v0.5.11 and dstack-sdk 0.1.3:

mkosi-os-v0.6.0-rc0   Pre-release   mkosi-os-v0.6.0-rc0

The last release had to be renamed by hand afterwards to read as a dstack guest OS release, which is not something a publish path should require.

Fix

Title the release dstack OS v$VERSION instead of echoing the tag. The tag itself is unchanged, so the trigger, the mkosi-os-v* tag-format check and docs/rc-testing-runbook.md all stay as they are.

VERSION is the build job's output, read from os/mkosi/versions.env, and the preceding validation step already fails the job unless the tag equals mkosi-os-v$DSTACK_VERSION. So the title cannot drift from the tag or from the image's own metadata.json version.

Verification

• gh release list on this repo confirms the current title is the raw tag and that no other release advertises a build backend.
• Parsed the edited workflow with yaml.safe_load and re-read the Create the release step; the diff is the --title value plus a comment.
• Expanded the new title with the released version: VERSION=0.6.0 → dstack OS v0.6.0.

Release-notes body, assets, --verify-tag and --prerelease are untouched.