[FEATURE]: Distribute an OSS Slack adapter for AURA #752

Outcome

A user can deploy the supported open-source Slack adapter, provide their Slack app configuration and credentials, and interact with a self-hosted AURA deployment from Slack without writing integration code or requiring Mezmo-hosted infrastructure.

Scope

• Distribute the Slack adapter as an open-source repository.
• Support Slack Socket Mode as the OSS deployment path.
• Support direct messages, mentions, threads, and multi-turn conversation behavior.
• Map Slack conversations to AURA sessions through the shared adapter contract.
• Make deployment configuration-driven, including Slack credentials and the target AURA endpoint.
• Keep AURA usable from Slack without exposing the AURA runtime publicly.

Out of scope

• Slack Marketplace distribution.
• Mezmo-hosted ingress or workspace routing.
• Native managed installation or administration through the Mezmo commercial platform.