feat(llm): structured error taxonomy with a shared HarnessError base (RFC 005 pt 2)

Introduce HarnessError in dsh-llm (the leaf package): a stable machine-routable
code distinct from the message, cause chaining, name from the subclass, plus
isHarnessError. LlmError, ToolArgsError, and InvariantError now extend it.

Tool failures carry the structure end-to-end: ToolExecutionResult gains
error: { name, code } (populated from a thrown HarnessError), and the loop
forwards it onto the tool/result session event (which gained the same optional
field) for retry/sandbox plugins and replay. The loop's toError wraps non-Error
throws in a HarnessError(code: UNKNOWN, cause) instead of a bare Error.

Landed last and in isolation so it's a pure upgrade over the plain Error+code
the earlier PRs used — independently revertible. Graduates RFC 005 pt 2 ->
ADR 0015; RFC 005 now fully implemented.

yarn.lock

@@ -621,6 +621,7 @@ __metadata:
     cordis: "npm:^4.0.0-rc.6"
   peerDependencies:
     "@deepseek-ai/dsh-agent": ^0.0.1
+    "@deepseek-ai/dsh-llm": ^0.0.1
     "@deepseek-ai/dsh-session": ^0.0.1
     cordis: ^4.0.0-rc.6
   languageName: unknown