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/0015-structured-error-taxonomy.md

@@ -0,0 +1,24 @@
+# ADR 0015: Structured error taxonomy
+
+Status: accepted (2026-06-14)
+
+## Context
+
+Failures crossed seams as bare strings. A tool error flattened to a text block — name, code, and stack lost — so a future sandbox/retry plugin couldn't tell ENOENT from EACCES, and the model got less actionable feedback than it could. A non-Error throw degraded further: the loop wrapped it in `new Error(String(x))`, dropping any code. And `LlmError` was the only typed error in the system, with no shared base, so there was nothing for a consumer to `instanceof` against generically.
+
+This is the last of the RFC 005 pieces and the one the user was most skeptical of, so it was deliberately built **last and in isolation**: the earlier PRs (arg validation, dev invariants) threw plain `Error`s with a `code` field, decoupled from any shared base, so this change is a pure upgrade and is independently revertible without unpicking them.
+
+## Decision
+
+A single `HarnessError extends Error` base in `dsh-llm` (the leaf package every other imports — no new dependency edge): a stable `code` distinct from `message`, `cause` chaining via `ErrorOptions`, and `name` defaulting to the subclass. `isHarnessError` narrows at seams.
+
+- `LlmError`, `ToolArgsError` (dsh-tools), and `InvariantError` (dsh-invariants) now extend it, keeping their existing codes.
+- `ToolExecutionResult` gains optional `error: { name, code }`, populated in the registry's catch when the thrown value is a `HarnessError`. The agent loop forwards it onto the `tool/result` session event (which gained the same optional field), so the structured failure survives into the log for retry/sandbox plugins and replay. The model-facing text block is unchanged.
+- The loop's `toError` wraps a non-Error throw in a `HarnessError` (`code: 'UNKNOWN'`, original chained as `cause`) instead of a bare `Error`, so even a bad throw carries a routable code into the session `error` event (which already surfaced `code`).
+
+## Consequences
+
+- Errors are machine-routable end-to-end: a plugin can branch on `error.code` rather than substring-matching a message.
+- One base class is imported widely, but it lives in the package everyone already depends on, so the cost is a single import, not a new edge.
+- `deriveMessages` does not surface `error` into model history — the model still sees the text block; the structured field is for code and replay.
+- Reverting this PR returns the earlier errors to plain `Error`+`code` form; nothing else in the stack depends on the shared base.

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 |

docs/rfc/005-runtime-validation-and-error-taxonomy.md

@@ -1,6 +1,6 @@
 # RFC 005: Runtime validation at the model boundary, error taxonomy, dev-mode invariants
 
-Status: partially implemented — part 1 (arg validation) → [ADR 0011](../adr/0011-runtime-arg-validation.md); part 3 (dev invariants) → [ADR 0012](../adr/0012-dev-invariants-over-deep-readonly.md); part 2 (error taxonomy) in progress
+Status: implemented — part 1 (arg validation) → [ADR 0011](../adr/0011-runtime-arg-validation.md); part 3 (dev invariants) → [ADR 0012](../adr/0012-dev-invariants-over-deep-readonly.md); part 2 (error taxonomy) → [ADR 0015](../adr/0015-structured-error-taxonomy.md)
 
 ## Problem
 

docs/rfc/README.md

@@ -8,7 +8,7 @@ Proposals for substantial future work — reviewed before implementation, unlike
 | [002](002-mutation-testing.md) | Mutation testing as the coverage counterweight | proposed |
 | [003](003-deterministic-and-stress-testing.md) | Deterministic tests + replay invariant fixture + race stress | proposed |
 | [004](004-architectural-conformance.md) | Architectural rules: dependency-cruiser, adapter conformance kit | proposed |
-| [005](005-runtime-validation-and-error-taxonomy.md) | Runtime arg validation, structured error taxonomy, dev-mode invariants | partially implemented |
+| [005](005-runtime-validation-and-error-taxonomy.md) | Runtime arg validation, structured error taxonomy, dev-mode invariants | implemented |
 | [006](006-doc-sync-and-api-reports.md) | Doc-sync enforcement and API extractor reports | implemented (pts 1-2; pt 3 deferred) |
 | [007](007-supply-chain-and-vendor-drift.md) | Supply chain checks and vendor drift verification | proposed |
 | [008](008-immutable-public-surfaces.md) | Deep-readonly public surfaces | implemented (revised) |