Use explicit ts specifiers for declarations

Restore explicit .ts relative specifiers in source and enable rewriteRelativeImportExtensions so emitted JS uses .js while declarations keep explicit .ts specifiers. Add a NodeNext declaration-consumer gate to prevent extensionless declaration regressions.

packages/core/tools/src/index.ts

@@ -24,7 +24,7 @@ export {
   type InferArgs,
   type DefineToolOptions,
   type JsonSchemaObject,
-} from './schema'
+} from './schema.ts'
 
 declare module 'cordis' {
   interface Context {

packages/core/tools/src/schema.ts

@@ -21,7 +21,7 @@
 
 import type { ContentBlock } from '@deepseek-ai/dsh-llm'
 import { assertNever, HarnessError } from '@deepseek-ai/dsh-llm'
-import type { ToolCallPresentation, ToolDefinition, ToolExecution, ToolResult, ToolResultPresentation } from './index'
+import type { ToolCallPresentation, ToolDefinition, ToolExecution, ToolResult, ToolResultPresentation } from './index.ts'
 
 // ---------------------------------------------------------------------------
 // SchemaSpec — the author-facing per-property type