chore: add plugin publication baseline

[RomeoRaven]

Summary

• add an explicit MIT license for this independently distributed plugin
• publish repository and protoAgent homepage provenance in the plugin manifest
• declare httpx as a host-scoped dependency because the plugin imports it in-process
• add least-privilege Linux, Windows, and macOS CI for lint, format, and tests
• document the evidence boundary and regression-test publication metadata

Public scope

Files: LICENSE, .github/workflows/ci.yml, README.md, PROTO.md, protoagent.plugin.yaml, tests/test_packaging.py.

This is shared repository publication hygiene for an external protoAgent plugin. It contains no installation-specific paths, credentials, runtime data, or private policy.

Verification

• ruff check .
• ruff format --check .
• pytest -q — 16 passed
• manifest parsed by the accepted S1 protoAgent loader with httpx resolved to host scope

Boundaries

No plugin behavior, live configuration, release/tag, deployment, compatibility-version claim, Dependabot setting, or branch rule changes.