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.

vitest.e2e.config.ts

@@ -0,0 +1,34 @@
+import tsconfigPaths from 'vite-tsconfig-paths'
+import { defineConfig } from 'vitest/config'
+
+// Real-API end-to-end tests: `yarn test:e2e`, file pattern *.e2e.ts.
+// Separate from the default suite (`yarn test`, *.spec.ts) on purpose —
+// these hit the live DeepSeek API, spend tokens, and need a key.
+//
+// Secrets: tests gate themselves with
+// `describe.skipIf(!process.env.DEEPSEEK_API_KEY)`, so the suite passes
+// (all-skipped) without credentials — CI has none and stays green. Put the
+// key in the environment or in a gitignored `.env` at the repo root:
+//
+//     DEEPSEEK_API_KEY=sk-…
+//     DEEPSEEK_BASE_URL=https://…   # optional, defaults to the public API
+try {
+  // Node >= 21.7 native; throws when the file does not exist.
+  process.loadEnvFile(new URL('.env', import.meta.url).pathname)
+} catch {
+  // No .env — fine, the environment may already carry the variables.
+}
+
+export default defineConfig({
+  // Same resolution note as vitest.config.ts: bare workspace names resolve
+  // through the root tsconfig paths map; the native option cannot do this.
+  plugins: [tsconfigPaths({ projects: ['./tsconfig.test.json'] })],
+  test: {
+    include: ['packages/*/tests/**/*.e2e.ts', 'examples/*/tests/**/*.e2e.ts'],
+    // Real model calls: generous timeouts, one retry for transient flakes,
+    // no coverage (unit suites own the coverage gate).
+    testTimeout: 120_000,
+    hookTimeout: 30_000,
+    retry: 1,
+  },
+})