refactor: unify CLI session flow and simplify settings domain

[LIU9293]

Summary

• centralize CLI thread session create/reuse logic with createCliThreadSessionManager and migrate all CLI providers to use it
• remove dead code paths (Redis session write APIs, unused selection-reply runtime, and legacy Slack OAuth compatibility surface)
• extract shared settings-domain helpers and reuse them across Slack, Discord, and Lark settings implementations

Validation

• bun run typecheck
• bun test