fix(ci): Prevent release-please double-run on release PR merge

[WilliamBerryiii]

Bug Description

When a release-please PR merges to main, the push event triggers the CI workflow (main.yml), which runs the release-please job again. This causes release-please to immediately create a spurious follow-up release PR with duplicate changelog entries.

Example: PR #143 (chore(main): release 0.1.0) merged → triggered CI → release-please ran again → created PR #144 (chore(main): release 0.2.0) with duplicate content.

Steps to Reproduce

1. Merge a release-please PR to main
2. Observe the CI workflow triggers on the push to main
3. The release-please job runs and creates another release PR

Expected Behavior

The release-please job should skip when the triggering commit is a release-please merge commit (chore(main): release ...).

Root Cause

The original standalone release-please.yml (commit b75ccaa) included a skip guard:

if: "${{ !startsWith(github.event.head_commit.message, 'chore(main): release') }}"

This was lost when the job was consolidated into main.yml (commit f930b6b).

Fix

Restore the if condition on the release-please job in .github/workflows/main.yml.