simplify(llm): drop unconsumed adapter-change event and assembled call surfaces

The LLM service exposed three call surfaces (stream/streamBlocks/generate) but
the only production consumer — the agent loop — uses stream() exclusively,
feeding raw chunks through its own BlockAssembler for replay fidelity. Drop the
speculative convenience surfaces and the registry-change event that no listener
consumed, leaving stream() as the single model-call contract for both
production and tests.

- Remove LlmService.streamBlocks() and generate(), the llm/generate waterfall,
  and GenerateResult.
- Remove the llm/adapter-change event (declaration + emits) and the
  listener-throw rollback ordering that existed only to protect it; keep the
  HMR rollback disposer.
- Remove BlockAssembler.flushReady()/flushRemaining()/result() and the flushed
  cursor — the streaming-flush slice existed only for streamBlocks().
- Adapter tests drive a stream()+BlockAssembler helper (tests/assemble.ts)
  instead of generate(), exercising the same path production uses.
- Land the AGENTS.md "RFCs are proposals, not golden truth" principle and move
  both RFCs proposed -> implemented.

Implements:
- docs/rfc/implemented/simplification/2026-06-20-drop-unconsumed-llm-adapter-change-event.md
- docs/rfc/implemented/simplification/2026-06-20-drop-unconsumed-llm-assembled-surfaces.md

packages/llm/llm-pi-ai/tests/assemble.ts

@@ -0,0 +1,26 @@
+/**
+ * Test helper: drive `ctx.llm.stream()` through a `BlockAssembler` and return
+ * the assembled message + usage + finish reason. This exercises the same
+ * streaming path production uses (the loop), rather than a service-level
+ * one-shot convenience method.
+ */
+
+import { BlockAssembler } from '@deepseek-ai/dsh-llm'
+import type { Context } from 'cordis'
+import type { FinishReason, GenerateOptions, Message, TokenUsage } from '@deepseek-ai/dsh-llm'
+
+export interface AssembledResult {
+  message: Message
+  usage?: TokenUsage
+  finish: FinishReason
+}
+
+export async function assemble(ctx: Context, options: GenerateOptions): Promise<AssembledResult> {
+  const assembler = new BlockAssembler()
+  for await (const chunk of ctx.llm.stream(options)) assembler.push(chunk)
+  return {
+    message: assembler.message(),
+    ...assembler.usage !== undefined ? { usage: assembler.usage } : {},
+    finish: assembler.finish,
+  }
+}