ci(release): tag automatically when a dev-v* branch merges to main

[roncodes]

Adds the release-tag caller. Part of the rollout for fleetbase/fleetbase#620, which holds the reusable workflow this delegates to.

What it does

When a dev-v* branch is merged to main, it validates and pushes the tag — and nothing else. create-release.yml and the publish jobs in this repository already chain off push: tags: ['v*'], so a second release path would duplicate them.

The version is never retyped. It comes from the branch name (dev-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 dev-v<semver> branch
• every version file exists, yields a version, and matches
• RELEASE.md exists, is non-empty, and its first line names this version
• 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 main at run time.

Before this can release

1. ci(release): tag automatically when a dev-v* branch merges to main fleetbase#620 must merge first — @main will not resolve until it does.
2. This repo needs a RELEASE.md whose first line names the version, e.g.

   > v1.2.3 ~ "What this release is about"
   

   Without it the notes check fails and no tag is pushed. That check is the only thing that can distinguish notes written for this release from the previous release's notes nobody replaced.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fa37e78) to head (da92ac0).

Additional details and impacted files

@@             Coverage Diff             @@
##                main      #297   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity      9766      9766           
===========================================
  Files            521       521           
  Lines          37762     37762           
===========================================
  Hits           37762     37762           

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

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