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.

docs/adr/README.md

@@ -26,3 +26,4 @@ Do NOT write an ADR for: a mechanical or local choice (a variable name, a one-fi
 | [0012](0012-dev-invariants-over-deep-readonly.md) | Dev-mode invariants over compile-time deep-readonly | accepted |
 | [0013](0013-property-based-testing.md) | Property-based testing for protocol-shaped code | accepted |
 | [0014](0014-doc-sync-enforcement.md) | Doc-sync enforcement (doc code blocks + event taxonomy) | accepted |
+| [0015](0015-structured-error-taxonomy.md) | Structured error taxonomy (HarnessError base) | accepted |