Coalesce commentary Thinking task updates

[decofe]

Summary

• reuse one stable commentary Thinking task id instead of creating one task per commentary message
• show cumulative commentary details on that task
• default Slack task streams to native dense display mode to reduce task-card pressure
• add/update regression coverage for commentary coalescing and Slack stream payload shape

Test plan

• bun test src
• bun run typecheck
• bun test test/chat-sdk-emulate.test.ts
• bun run check:types

Prompted by: @Rjected
Prompted by: @gakonst