fix: skip duplicate release events for tagged main

[cablate]

Problem

After #89 released v0.0.57, a second push run for the same merge commit started seven minutes later. It fetched the already-tagged main and failed in Update CHANGELOG because there were no unreleased commits. No duplicate npm publish occurred, but this left a misleading failed release run and repeated live E2E work.

Change

Add a lightweight check job at the start of the serialized release workflow. If the latest main commit already has a version tag, skip the publishing job; otherwise preserve the existing release path. This handles stale duplicate push events without relying on an empty changelog command to fail.

Verification

• On current tagged main, git describe --exact-match returns v0.0.58 (skip case)
• On the new untagged PR commit, the exact-match check fails (release case)
• Workflow YAML parsed successfully and git diff --check passed

The first hosted verification of the release case will occur when this PR merges; no npm or Registry publication was performed by this PR.