[grok] CONTRACT: list OpenWrtPlugin; plugins package __init__ empty

[sfloess]

[grok] Post-verification nits @ 0.9.

Problems

1. CONTRACT.md "Recommended Imports for App v1" lists plugins.static but not plugins.openwrt, even though OpenWrtPlugin is a first-class built-in.
2. tftpos/plugins/__init__.py is empty (0 bytes). Re-exports of FirmwarePlugin, StaticFirmwarePlugin, OpenWrtPlugin would make the package surface clearer for apps (from tftpos.plugins import StaticFirmwarePlugin).

Acceptance criteria

• CONTRACT recommended imports / public API mention OpenWrtPlugin (core or extended — pick one and be consistent with SCOPE)
• tftpos/plugins/__init__.py exports the public plugin types (or document that imports must be from submodules)
• Align with __init__.py package docstring

Notes

Small consistency pass for app authors.