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/adr/README.md

@@ -22,3 +22,4 @@ Do NOT write an ADR for: a mechanical or local choice (a variable name, a one-fi
 | [0008](0008-tsdown-over-dumble.md) | tsdown for JS bundling instead of dumble | accepted |
 | [0009](0009-capability-seams.md) | Capability seams — interface / implementation / consumer split | accepted |
 | [0010](0010-twin-llm-adapters.md) | Two LLM adapters as a design-verification twin | accepted |
+| [0011](0011-runtime-arg-validation.md) | Runtime arg validation at the model boundary | accepted |