fix(plugin): bind adapter to real Hermes gateway API; add contract-test gate

- Resolve the Hermes gateway API lazily via get_adapter_class() so the adapter
  always inherits the real gateway.platforms.base.BasePlatformAdapter instead
  of the ImportError fallback stub (root cause of the AttributeError:
  'set_message_handler' crash at gateway startup).
- Pass Platform('meshtastic') enum to super().__init__ and align
  connect(*, is_reconnect=False) -> bool with the gateway contract.
- Split pure formatting utils into formatting.py; remove the stub fallback.
- Add tests/contract (L0-L3): real-base binding, ABC instantiation, gateway
  run.py wiring replay, signature and event-shape conformance, run against
  hermes-agent pinned to the deployed image source (v2026.8.31).
- Add scripts/contract-test.sh and a contract-test CI job; release publish
  now requires contract tests to pass.
- Untrack stray __pycache__ artifacts.

plugin/.gitignore

@@ -0,0 +1,6 @@
+dist/
+__pycache__/
+*.pyc
+.venv-contract/
+.hermes-src/
+.pytest_cache/