Add two DeepSeek LLM adapters: dsh-llm-deepseek and dsh-llm-pi-ai

The first real LlmAdapter implementations, shipped as a deliberate pair:
same models and wire protocol, completely different internals, so the
StreamChunk protocol is verified across independent implementations.

- dsh-llm-deepseek: hand-rolled fetch + SSE parser + chunk-translation
  state machine against the official chat-completions format (thinking
  mode via top-level thinking/reasoning_effort; the empty-string
  reasoning_content first chunk; usage attached to the finish chunk or
  trailing; reasoning_content passback on tool-call turns; disjoint
  cache-token accounting).
- dsh-llm-pi-ai: the same endpoint through @earendil-works/pi-ai,
  mapping its event vocabulary (parsed tool arguments, in-stream error
  events, folded reasoning tokens) onto the same chunks.

The agent loop now honors the in-band error path: an adapter that ends
its stream with finish {kind:error|aborted} (the only option for
adapters that can't throw mid-stream, like pi-ai) is translated into a
step error, so the turn ends error/aborted with a logged error event
instead of a normal completed assistant message. This makes the
StreamChunk error contract real for both adapters; docs/architecture.md
and the StreamChunk doc are updated accordingly.

New yarn test:e2e (vitest.e2e.config.ts, *.e2e.ts) runs key-gated
real-API matrices for both adapters across V4 Flash/Pro and all
thinking/effort levels; it self-skips without DEEPSEEK_API_KEY. Unit
suites run against local node:http mock SSE servers at 100% per-file
coverage.

packages/llm/src/index.ts

@@ -30,9 +30,14 @@ declare module 'cordis' {
   }
 }
 
-/** Typed error for LLM-related failures. The `code` string enables programmatic handling. */
+/**
+ * Typed error for LLM-related failures. The `code` string enables programmatic
+ * handling (e.g. `AUTH`, `RATE_LIMIT`, `NO_ADAPTER`); `status` carries the HTTP
+ * status when the error originated from a non-2xx provider response (absent for
+ * protocol/usage errors that have no HTTP status).
+ */
 export class LlmError extends Error {
-  constructor(message: string, public code: string) {
+  constructor(message: string, public code: string, public status?: number) {
     super(message)
     this.name = 'LlmError'
   }
@@ -45,8 +50,10 @@ export class LlmError extends Error {
  * StreamChunk) and one provider's wire format. Adapters register themselves
  * via `ctx.llm.registerAdapter(models, adapter)`.
  *
- * TODO: the first real adapter (DeepSeek V4) lands in a later phase; until
- * then only mock adapters (tests, demo) exist.
+ * Real implementations: `@deepseek-ai/dsh-llm-deepseek` (hand-rolled
+ * fetch/SSE) and `@deepseek-ai/dsh-llm-pi-ai` (pi-ai-backed) — two
+ * deliberately different internals over the same contract; see the
+ * adapter contract documented on `StreamChunk` in `./types.ts`.
  */
 export abstract class LlmAdapter {
   /** Stream one model call as raw chunks. The only required method. */