Automate per-branch releases (dev prereleases, main stable)

[imedboumalek]

What

Replaces dev-release.yaml with auto-release.yaml, which runs on every push to dev and main and handles tag + publish automatically.

Push to	Publishes	Version
dev	prerelease	pubspec X.Y.Z → X.Y.Z-dev.N (N = highest existing dev tag + 1, auto)
main	stable	pubspec version verbatim (keeps +build)

• The pubspec holds the target X.Y.Z; -dev.N is never hand-edited. Since pub.dev publishes the pubspec version at the tag and requires tag == v<version>, the dev path synthesizes a release commit setting version: X.Y.Z-dev.N and tags that (tag-only push; dev branch untouched → no trigger loop).
• main is guarded: if vX.Y.Z already exists it's a no-op, so only a version bump releases.
• Publish still runs via publish.yaml, dispatched on the new tag ref.

Rollout (expected, agreed)

• First merge to dev publishes a stray 0.4.0-dev.1 (pubspec base is 0.4.0) — harmless, sorts below 0.4.0, retractable.
• First merge to main auto-publishes 0.4.0+1 (the README banner release), since that tag doesn't exist yet. This is how the banner reaches pub.dev.

Notes

• The dev path tags a non-branch (synthetic) commit; pub.dev OIDC verifies tag ref + repo, not branch-reachability, so this is expected to work — the first real dev prerelease confirms it.
• No CHANGELOG entry per -dev.N (minor pana hit, accepted for the dev channel).