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.

.agents/notes/rejected/simplification/2026-07-26-dependency-swaps-rejected-by-nih-audit.zh.md

@@ -18,7 +18,7 @@ Status: rejected — 下列每一项替换在证据上都未达到净简化门
 - **以 `vscode-languageserver-types` 承担 lsp-local 的协议类型子集**：约 80 行类型加约 45 行守卫，但上游守卫在两个方向上都与本仓库不一致（接受本仓库必须拒绝的 `uri: undefined`；强制要求本仓库容忍缺失的 `targetRange`），而且 initialize 结果的形状住在 `vscode-languageserver-protocol` 里，会把 `vscode-jsonrpc` 拖成运行时依赖——为 80 行严格贴合规范的代码付出约 1 MB。
 - **以 `json-rpc-2.0` 替换 `dsh-jsonrpc`**：可删除的关联/分发代码确实存在（约 100–130 行），但 NDJSON 协议格式（wire format）必须与手写的 Python SDK 客户端逐位一致，该包只有单一维护者，且 [GUI RPC 决策](../../implemented/architecture/2026-07-19-gui-layering-and-rpc-protocol.md)已把这个包当作冻结的窄接口面对待。`vscode-jsonrpc` 更不合适（Content-Length 分帧、该协议并不具备的取消词汇）。
 - **以 `jsonrpcclient` 承担 Python SDK 客户端**：v4 只做消息的构造/解析——约 20 行——而真正要紧的 500 行（子进程生命周期、线程化读取器、id 关联、双向的服务端角色应答）全都保留；该库处于低维护模式。
-- **以 `eventsource-parser` 替换 apiproxy 的 `readSse`**：可删除的分帧只有约 15 行，线路两端都在仓库内，规范符合性无关紧要，而且这会给一个浏览器安全的包添加依赖。（对比 [llm-deepseek 提案](../../proposed/simplification/2026-07-26-eventsource-parser-for-deepseek-sse.md)：那里线路对面是真实的提供方。）
+- **以 `eventsource-parser` 替换 apiproxy 的 `readSse`**：可删除的分帧只有约 15 行，线路两端都在仓库内，规范符合性无关紧要，而且这会给一个浏览器安全的包添加依赖。（对比 [llm-deepseek 提案](../../implemented/simplification/2026-07-26-eventsource-parser-for-deepseek-sse.md)：那里线路对面是真实的提供方。）
 
 **重试、定时器与异步：**