Add manual release dispatch workflow

[pengfei-threemoonslab]

Summary

• add workflow_dispatch support to the Release workflow with a required version input
• validate requested releases against pyproject.toml and src/agents_shipgate/__init__.py, then create an annotated release tag from the default branch in the same approved run
• verify GitHub Release, PyPI, and GitHub Marketplace visibility after publishing
• document the manual release path and pin it with action metadata tests

Validation

• python -m ruff check .
• YAML parse check for .github/workflows/release.yml
• python -m pytest tests/test_action_metadata.py tests/test_public_surface_contract.py tests/test_docs_links.py -> 254 passed

Notes

• The job still supports the existing push.tags: v* release path.
• Marketplace visibility is checked with retry/backoff and emits a warning if GitHub has not refreshed it within the retry window.