isError results replace thrown ToolInputError (pi 1.0 convention)

Authoring refinements (quizInputProblems, nodeId checks, ask/explain
refinements, submit-agent pipeline laws) now return invalidInputResult —
same model-facing failure and retry semantics pi minted from the throw,
but details keeps {tool, problems} as data instead of the throw path's
empty {}. ToolInputError deleted; wire shape unchanged (verified against
pi's own thrown-error conversion in agent-loop).

src/agent/subagents/practice.ts

@@ -4,7 +4,7 @@
  * Three single-submit workers on the shared runSubmitAgent seam (one
  * TypeBox-validated submit call each; a pi agent purely so the model can
  * retry a schema-invalid or LAW-violating call — the validate hook throws
- * ToolInputError at the execute seam and pi turns it into an error tool
+ * an invalidInputResult isError result at the execute seam (pi 1.0 convention)
  * result the model sees).
  *
  * Roles: partitioner | problemGenerator | grader — all strong-tier