ci: skip-existing on PyPI publish to unblock concurrent PR pushes

When a PR is pushed multiple times in quick succession (or two CI runs
race), the version-bump step can compute the same next version twice
because PyPI's JSON API has a small cache window after a release.
The second 'Publish to PyPI' then fails with HTTP 400 "File already
exists", marking the entire CI run red and blocking PR merge even
though tests, type-check, build and verify all passed.

Add skip-existing: true so the publish action treats an existing
version as a non-error (it logs a warning and exits 0). PR merge is
no longer gated by this race.

Side note: skip-existing means the second push's *content* won't be
republished under the same version. The proper long-term fix is to
embed the commit SHA in the dev version (e.g. 0.0.187+sha.<short>)
so each push gets a unique artefact. Tracked separately.

release-test.yml is intentionally left unchanged: it is triggered
manually with an explicit version bump, where a conflict should
fail loudly rather than be silently skipped.

Change-Id: I4be2fb05fd06e32b83dd64d52bd62bfe8b8355cf
Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: OhYee <oyohyee@oyohyee.com>

Author: OhYee

.github/workflows/ci.yml

@@ -163,6 +163,10 @@ jobs:
       with:
         password: ${{ secrets.PYPI_API_TOKEN }}
         verify-metadata: false
+        # 当 PR 多次 push 或并发 CI 跑出相同版本号时，跳过已存在版本而非报错。
+        # 根因是版本号自增逻辑依赖 PyPI API 拉取最新版本，存在缓存/并发窗口。
+        # 不阻塞 PR 合入即可，长期方案是在版本号中嵌入 commit SHA 实现唯一化。
+        skip-existing: true
 
     - name: Create and push tag
       if: steps.changes.outputs.agentrun_changed == 'true'