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/assembler.ts

@@ -6,6 +6,7 @@
  */
 
 import { CallId } from './brand.ts'
+import { assertNever } from './never.ts'
 import type { ContentBlock, FinishReason, GenerateResult, Message, StreamChunk, TokenUsage } from './types.ts'
 
 interface PartialBlock {
@@ -82,6 +83,7 @@ export class BlockAssembler {
         this._finish = chunk.reason
         return
       }
+      default: return assertNever(chunk, 'BlockAssembler.push')
     }
   }