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/sdk/helper/src/features/builtin/provider.ts

@@ -1,5 +1,5 @@
 /**
- * Required hand-rolled DeepSeek and custom pi-ai provider behavior.
+ * Required direct-fetch DeepSeek and custom pi-ai provider behavior.
  *
  * @module @deepseek-ai/dsh-helper/features/builtin/provider
  */
@@ -74,7 +74,7 @@ export class ProviderFeature extends ExclusiveOptionFeature {
   override readonly required = true
   override readonly options = [new DeepSeekOption(), new CustomOption()]
 
-  /** Prefer the hand-rolled adapter and its public endpoint defaults. */
+  /** Prefer the direct-fetch adapter and its public endpoint defaults. */
   override defaultOptions(): readonly string[] {
     return ['deepseek']
   }