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.md

@@ -12,7 +12,7 @@ DeepSeek Harness SDK is a plugin-based agent harness on vendored Cordis: **every
 vendor/      Vendored Cordis source — manifest + sync procedure in vendor/README.md
 packages/    @deepseek-ai/dsh-<pkg> workspaces at packages/<group>/<pkg>/
   core/        product API spine: session, system-prompt, tools, agent, agent-loop
-  llm/         LLM seam + DeepSeek adapters (hand-rolled + pi-ai design twin)
+  llm/         LLM seam + DeepSeek adapters (direct-fetch + pi-ai design twin)
   bash/        bash executor seam + local impl + model-facing bash tools
   pty/         persistent PTY seam/backend/tools
   fs/          filesystem seam + local impl + policy gate + read/write/edit tools