feat: add Slack channel integration

[furukama]

Summary

HybridClaw currently ships channel integrations for Discord, Microsoft Teams, iMessage, WhatsApp, and email. Slack is still missing and would be a strong workspace-facing addition.

Why

Slack is a natural fit for team-facing assistant workflows, approvals, scheduled jobs, and channel- or thread-scoped sessions.

Proposed scope

• Add a Slack channel runtime under src/channels/slack/.
• Support setup for bot token/app credentials through CLI/onboarding.
• Map Slack DM, channel, and thread contexts to canonical HybridClaw sessions.
• Support inbound text, files, and thread replies.
• Support outbound replies, thread-aware delivery, and slash-command or app-command entrypoints.

Candidate UX

• hybridclaw channels slack setup ...
• per-workspace installation and auth guidance
• slash command and mention-based invocation options

Implementation notes

• Preserve HybridClaw's current session routing model while adapting to Slack threads.
• Keep the first version narrow: one reliable event ingestion path and one reliable delivery path.
• Treat workspace/channel permissions carefully so the agent does not overreach by default.
• Reuse the existing attachment/media normalization approach from other channel adapters.

Acceptance criteria

• Users can connect a Slack workspace and have stable end-to-end messaging.
• DM, channel, and thread routing behave predictably.
• Slash-command or app-command invocation is supported in the first release.
• Documentation covers setup, scopes, routing, and deployment caveats.
• Tests cover inbound event handling, thread routing, and outbound delivery.