Release v0.1.9

✨ Highlights

• Provider Plugin API completed: plugin SDK now supports register_providers in addition to register_tools and register_channels.
• Plugin-aware provider bootstrap: runtime command paths now honor plugin provider factories.
• Version metadata sync: backend package metadata moved to 0.1.9.

➕ Added

• Plugin SDK provider hook: register_providers.
• Provider factory helpers: collect_provider_factories, has_provider_factory, build_provider.
• Plugin docs updated with provider-hook usage.

🔄 Changed

• CLI runtime commands (gateway, agent, digest) now resolve plugin provider factories before API-key preflight failure.
• Embedded Agent bootstrap now accepts plugin-registered provider factories.
• g-agent plugins list now shows providers hook visibility.

🐛 Fixes

• Removed remaining provider-extension gap where custom providers required core code edits.

📚 Notes

• This release keeps historical v0.1.8 bridge-hardening notes unchanged.