fix(session): type turn/end error as one structured failure

TurnEndReasonMap.error now carries a single `error: LlmFailure` field:
an LlmError keeps its structured facts, any other error flattens to
errorChain text under the UNKNOWN code. Consumers read message/code
directly instead of defending against an unknown union — this also fixes
errorChain() rendering structured failures as '[object Object]' in the
TUI and ACP error paths. Document the turn-stopping contract: a
concludesTurn result never short-circuits already-submitted next-step
work (same-step additionalContexts or racing steering still runs), data
decides.

packages/core/session/src/types.ts

@@ -4,6 +4,7 @@ import type {
   CallId,
   LlmCallConfig,
   LlmCallConfigAdapterDefaults,
+  LlmFailure,
   StreamChunk,
   TokenUsage,
   ToolResultMessage,
@@ -109,9 +110,11 @@ export interface TurnEndReasonMap {
 
   blocked: { kind: 'blocked' }
   /**
-   * The turn failed.
+   * The turn failed. `error` is always a structured failure: the `LlmError`
+   * facts verbatim, or `{ message: errorChain(error), code: 'UNKNOWN' }`
+   * flattened from any other error.
    */
-  error: { kind: 'error'; error: unknown }
+  error: { kind: 'error'; error: LlmFailure }
   /** At least one step reached its output-token ceiling, even if a plugin continued the turn. */
   'max-tokens': { kind: 'max-tokens' }
   /**