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.

packages/tools/src/index.ts

@@ -15,6 +15,8 @@ import type {} from '@deepseek-ai/dsh-system-prompt'
 export {
   defineTool,
   schemaSpecToJsonSchema,
+  validateArgs,
+  ToolArgsError,
   type SchemaSpec,
   type SchemaProp,
   type SchemaType,