feat(x): add setup helpers and remove classic direct messages

[dancer]

summary

• remove classic direct message support from the base X adapter and direct users to the XChat adapter at @chat-adapter/x/chat
• add the @chat-adapter/x/setup subpath for one-time webhook registration and Activity API subscription management
• support OAuth 1.0a signing with query and form-encoded parameters, with OAuth 2.0 user-context fallback for subscription creation
• keep the adapter runtime on OAuth 2.0 and make the bot user ID optional until an operation such as liking requires it
• update package metadata, the adapter catalog, feature tables, setup guidance, and the historical DM payload note

test plan

• build chat and @chat-adapter/x
• run the X adapter test suite
• typecheck chat, @chat-adapter/x, and create-chat-sdk
• run the adapter integration suite
• run Ultracite on affected source files and run Konsistent
• build the documentation site
• verify the package contains and exports @chat-adapter/x/setup
• verify captured JSON payloads in sample-messages.md remain unchanged

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Project	Deployment	Actions	Updated (UTC)
chat	Ready	Preview, v0	Aug 10, 2026 6:56pm
chat-sdk-nextjs-chat	Ready	Preview	Aug 10, 2026 6:56pm