feat(tools): validate model-generated tool args at the boundary (RFC 005 pt 1)

defineTool now runs validateArgs against the SchemaSpec before execute, so a
malformed model call returns a self-correctable isError result listing the
violations instead of reaching the typed body untyped-in-practice. The
validator mirrors schemaSpecToJsonSchema semantics exactly (required from
required:true only, extra keys allowed, default not applied, object/array
without properties/items only type-checks, enum membership).

tool-bash's hand-rolled type/required checks (carrying the TODO(RFC 005)
stopgap note) are slimmed to just the value constraints the DSL can't express
(non-empty strings, positive timeout). Graduates RFC 005 pt 1 to ADR 0011.

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: proposed
+Status: partially implemented — part 1 (arg validation) → [ADR 0011](../adr/0011-runtime-arg-validation.md); parts 2-3 in progress
 
 ## Problem