feat: register plugin slash commands declaratively

[offendingcommit]

Summary

• add a public @command decorator for sync and async Hermes slash commands
• register decorated commands through register_plugin and expose them in RegistrationSummary
• validate command metadata, keep logs payload-safe, and document the new API
• verify registration and async dispatch against current Hermes Agent source

Validation

• make test
• make test-contract
• make build
• uv lock --check
• git diff --check