refactor(llm-deepseek): replace hand-rolled SSE parser with eventsource-parser

Implements the approved simplification Agent Note: sse.ts now pipes the
response body through TextDecoderStream and EventSourceParserStream
(eventsource-parser/stream) and keeps only the DeepSeek protocol shim —
yield each event's data, terminate on [DONE], throw
LlmError('STREAM_CLOSED') on EOF without the sentinel. The SSE
spec-conformance tests are deleted; sse.spec.ts pins only the
[DONE]/STREAM_CLOSED/EOF contract, including the new spec-strict verdict
that an unterminated trailing event is truncation (the old parser
flushed it — a robustness nicety no real provider shape needs).

eventsource-parser@^3.1.0 becomes llm-deepseek's second runtime
dependency (already in the lockfile transitively via the MCP SDK).

Docs: the Agent Note moves proposed/ → implemented/ and is rewritten per
the lifecycle contract; the rejected NIH roll-up note's inbound links
follow. The twin-adapters note, dsh-llm LlmAdapter JSDoc (and its
type-equiv fences), cookbook, group/package READMEs, root AGENTS.md
layout line, sdk-helper comments, and the regenerated config catalog
drop the "hand-rolled fetch + SSE" claim in both languages; all eight
touched pairs re-recorded.

packages/llm/llm-deepseek/README.zh.md

@@ -2,7 +2,7 @@
 
 [English](README.md) | 中文
 
-harness LLM seam 的 DeepSeek chat-completions 适配器：手写 `fetch` + SSE，将官方协议格式（真源：API 文档 guides/thinking_mode、guides/tool_calls、api/create-chat-completion）转换为 `StreamChunk` 协议。
+harness LLM seam 的 DeepSeek chat-completions 适配器：直接 `fetch` + SSE（由 `eventsource-parser` 分帧），将官方协议格式（真源：API 文档 guides/thinking_mode、guides/tool_calls、api/create-chat-completion）转换为 `StreamChunk` 协议。
 
 同一 seam 的第二个库支持实现位于 `@deepseek-ai/dsh-llm-pi-ai`。本包始终拥有 `deepseek` 提供方路由；在同一上下文中装载 `provider: deepseek` 的 pi-ai profile 会按设计抛出 `LlmError('DUPLICATE_ADAPTER')`。