Plugin: add monitor channels for cross-thread approvals and unread activity

[huntharo]

Summary

• add a /cas_monitor command that binds a conversation as a cross-thread monitor surface
• surface pending approvals, waiting-for-user-input prompts, and unread thread activity inferred from local last-seen state
• keep monitor bindings compatible with /cas_detach, /cas_status, and /cas_resume, and cover the new flow with unit tests

Notes

• unread activity is inferred from thread updatedAt against locally tracked last-seen timestamps because the app-server protocol does not expose a first-class unread bit
• this PR does not yet create provider-native jump targets for dedicated Discord threads or Telegram topics; it summarizes activity and points users back to /cas_resume <thread-id>

Fixes #19

Validation

• pnpm test
• pnpm typecheck