feat: add automatic version branch sync workflow based on PR labels

[anistark]

Adds automatic version branch syncing based on PR labels to streamline maintenance releases and backports.

Automatically syncs merged PRs to appropriate version branches based on labels:

Label mapping:

• patch → current version (v0.3.x)
• minor → next minor (v0.4.x)
• major → next major (v1.0.x)
• backport-v0.X.x → specific version branch
• No label → defaults to patch

Also, support specific version labels.

Example

PR merged with labels: [patch, backport-v0.2.x] → Syncs to v0.3.x (current) → Syncs to v0.2.x (explicit backport) → Comments on PR with results

Releases remain manual - this only handles keeping branches in sync.