Add Hermes agent support for local backend

[sm86]

Summary

Add first-class Hermes agent support when Contexto is configured to use a local backend instead of the hosted api.getcontexto.com service.

Context

@ekai/contexto already exposes a ContextoBackend boundary for swapping remote and local backends, but the plugin currently registers only RemoteBackend and requires an apiKey. Hermes should be able to ingest and retrieve context through the local backend path.

Proposed Work

• Add local backend configuration to the Contexto plugin.
• Allow Hermes agent metadata/events to flow through local ingest and search.
• Avoid requiring apiKey when local backend mode is enabled.
• Document setup for Hermes + local backend.
• Add tests or a smoke path covering local backend registration and Hermes event handling.

Acceptance Criteria

• Hermes can use Contexto with the local backend without hosted API credentials.
• Existing hosted backend behavior remains unchanged.
• Local backend errors are logged clearly.
• Docs include minimal configuration and run instructions.

[sm86]

#153