ci(release): tag automatically when a release branch merges

[roncodes]

Adds release-tag automation to this repository, using the release/v0.0.0 branch convention. Companion to fleetbase/fleetbase#641, which holds the reusable workflow this delegates to.

What it does

When a release branch is merged, it validates and pushes the tag — and nothing else. Whatever this repository already runs on push: tags: v* does the publishing, so there is no second release path.

The version is never retyped: it comes from the branch name (release/v1.2.3 ⇒ v1.2.3), and the tag is refused unless every version file and RELEASE.md already agree with it.

Validation, each failing rather than warning

• the PR was merged, not merely closed, from a release/v<semver> branch (legacy dev-v<semver> still accepted)
• every version file exists, yields a version, and matches
• RELEASE.md exists, is non-empty, and its first line names this version
• no RELEASE_NOTES_PLACEHOLDER marker remains
• no tag already exists at a different commit (same commit is a safe no-op)

The tag goes on the PR's merge commit, never on the default branch at run time.

Also in this PR

A seeded RELEASE.md. The workflow refuses to release without notes whose first line names the version — that check is the only thing that can distinguish notes written for this release from the previous release's notes nobody replaced. Replace both placeholder markers before your first release, or the tag is refused.

Merge order

fleetbase/fleetbase#641 first, since this references the reusable workflow at @main.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (95df8d2) to head (8aad28d).

Additional details and impacted files

@@             Coverage Diff             @@
##                main        #5   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       348       348           
===========================================
  Files             23        23           
  Lines           1470      1470           
===========================================
  Hits            1470      1470           

Flag	Coverage Δ
backend	100.00% <ø> (ø)

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