docs: add scheduled message support documentation

[bensabic]

• Add thread.schedule() and channel.schedule() to the Thread and Channel API reference pages (with ScheduledMessage type table)
• Add ScheduledMessage to the API overview exports table
• Add Scheduled messages subsection to the Threads, Messages, and Channels guide
• Mention schedule() in the NotImplementedError section of the error handling guide
• Add Scheduled messages row to the adapter feature matrix (Slack = Native, rest = No)
• Add Scheduled messages to the Slack adapter features table
• Add scheduleMessage() to the optional methods table in the Building a custom adapter guide
• Add schedule() to the Thread description in SKILL.md

Companion docs for #202.

[vercel]

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