chore(release): prepare 0.5.0

[dadachann]

Summary

• bump langbot-plugin to 0.5.0 after multi-tenant runtime merge
• add a tag-triggered PyPI publish workflow using the existing repository secret

Verification

• uv build produced both sdist and wheel
• PR feat(runtime): add isolated multi-tenant plugin workers #107 checks passed across Python 3.10/3.11/3.12, lint, package smoke, type checks, and protocol consistency

Release

After squash merge, tag 0.5.0; the publish workflow uploads to PyPI.