feat(service): add Slack Socket Mode channel

[edwin-zvs]

Summary

• add outbound Slack Socket Mode service channels for app mentions and direct messages
• route Slack threads through the shared service ingress runtime and post final replies with the bot token
• add prompt acknowledgement, event deduplication, reconnect backoff, workspace/channel allowlists, and live task reloads
• keep Slack tokens write-only while exposing credential-presence metadata
• add Slack channel configuration to the TUI and WebUI
• record the channel contract in spec 0174

Validation

• cargo build passes
• Slack transport, API mock, redaction, service ingress, supervisor, and TUI tests pass
• embedded WebUI JavaScript parses successfully with Node
• cargo test --workspace reached 1,305 passing CLI tests, then stopped on color::tests::truecolor_backend_still_emits_rgb; the same test fails unchanged on the top-level main checkout

Slack app setup

Enable Socket Mode, create an app token with connections:write, subscribe to app_mention and message.im, and grant the bot app_mentions:read, im:history, and chat:write.