Add assertNever with closed-vs-extensible exhaustiveness guidance

assertNever (dsh-llm) marks unreachable defaults on CLOSED unions:
adding a StreamChunk variant now breaks compilation at
BlockAssembler.push, and a value escaping its type at runtime throws
with diagnostics. The module doc and a new AGENTS.md convention spell
out the dividing line: merge-extensible unions (SessionEventMap,
ContentBlockMap, …) must NOT use assertNever — plugin-added variants
are valid unknown values there; handle known cases and fall through
with a comment.

packages/llm/src/index.ts

@@ -11,6 +11,7 @@ import type { ContentBlock, GenerateOptions, GenerateResult, StreamChunk } from
 import { BlockAssembler } from './assembler.ts'
 
 export * from './brand.ts'
+export * from './never.ts'
 export * from './types.ts'
 export { BlockAssembler } from './assembler.ts'