Expose stream options on thread.post

[notkadez]

Summary

• add thread.post(message, { stream: ... }) support for public stream options
• forward Slack stream presentation options like taskDisplayMode and stopBlocks
• update docs to use the public thread.post(...) API instead of undocumented thread.stream(...)

Testing

• corepack pnpm --filter chat test
• corepack pnpm --filter chat typecheck
• corepack pnpm --filter chat clean
• corepack pnpm --filter chat exec tsup
• copied docs into packages/chat/docs to match the package build step

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