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/implemented/simplification/2026-07-26-eventsource-parser-for-deepseek-sse.md

@@ -0,0 +1,28 @@
+# Agent Note: Replace the hand-rolled SSE parser in llm-deepseek with eventsource-parser
+
+Status: implemented
+
+English | [中文](2026-07-26-eventsource-parser-for-deepseek-sse.zh.md)
+
+## Problem
+
+`packages/llm/llm-deepseek/src/sse.ts` hand-implemented Server-Sent Events parsing: a streaming `TextDecoder`, event-block splitting on `\r?\n\r?\n`, `data:` payload extraction and joining, comment/field skipping, the `[DONE]` sentinel, a `STREAM_CLOSED` error on EOF without it, and a flush of a final unterminated event block. The file was ~67 lines with ~108 lines of dedicated tests (`tests/sse.spec.ts`) re-proving SSE spec behavior — UTF-8 split across chunks, CRLF handling, multi-`data:` joining, no-space-after-colon — that a maintained parser already guarantees. Its only consumer is `adapter.ts` (`yield* translate(parseSse(response.body))`).
+
+This is exactly the surface `eventsource-parser` owns: the de-facto standard SSE parser (it underlies the Vercel AI SDK and the MCP SDK), zero-dependency, actively maintained, and already present in this repo's lockfile transitively via `@modelcontextprotocol/sdk` — so adopting it directly adds no new supply-chain surface in practice.
+
+## Decision
+
+`sse.ts` delegates SSE framing to `EventSourceParserStream` from `eventsource-parser/stream`: `parseSse` pipes the response body through `new TextDecoderStream()` then `new EventSourceParserStream()` and keeps only the DeepSeek protocol shim — yield each event's `data`, terminate on `[DONE]`, and throw `LlmError('STREAM_CLOSED')` when the stream ends without the sentinel. All required builtins (`TextDecoderStream`, `pipeThrough`, async-iterable `ReadableStream`) exist at the Node ^22.19 engine floor. The spec-conformance tests are gone; `tests/sse.spec.ts` pins only the `[DONE]`/`STREAM_CLOSED`/EOF contract. `eventsource-parser` is `llm-deepseek`'s second runtime dependency after schemastery. The [twin-adapters note](../architecture/2026-06-13-twin-llm-adapters.md) and the `dsh-llm` JSDoc that branded this adapter "hand-rolled fetch + SSE parsing" now describe it as direct fetch with library-framed SSE.
+
+The library also strips a leading BOM (the hand-rolled parser would fail to match `data:` after one) and offers `maxBufferSize` hardening the hand-rolled parser lacked.
+
+## Alternatives considered
+
+- **Keep the hand-rolled parser.** Defensible under the [twin-adapters decision](../architecture/2026-06-13-twin-llm-adapters.md): the adapter is deliberately the hand-rolled design-verification twin of the pi-ai adapter. But that note's load-bearing distinction is owning the fetch/translate internals versus delegating to a full provider SDK; a ~700-byte SSE micro-parser is transport plumbing, not the design under verification. The twin-adapters note now states that reading explicitly.
+- **`createParser({onEvent})` callback API instead of the stream.** Works fed by a manual `TextDecoder` loop, but the `pipeThrough` composition deletes more of the hand-rolled code.
+
+## Consequences
+
+- The remaining shim only encodes the DeepSeek `[DONE]`/`STREAM_CLOSED` protocol; SSE framing edge cases are eventsource-parser's contract and are no longer re-proven here.
+- One deliberate robustness deviation is dropped: the hand-rolled parser flushed a final event block that lacked its terminating blank line, so a trailing `data: [DONE]` without `\n\n` still yielded DONE. eventsource-parser is spec-strict and only dispatches on the blank line, so that shape is now `STREAM_CLOSED`. Real providers and `dsh-llm-mock-server` always terminate events properly — the flush was a robustness nicety, not an observed provider shape — and `tests/sse.spec.ts` pins the new truncation verdict for that tail.
+- The documented "hand-rolled" identity of the twin adapter narrows to the fetch/translate internals; the twin-adapters note was updated in the same change rather than leaving the claim stale.