feat(tools): add ToolDefinition.timeoutMs declared+validated via defineTool

A tool declares its cooperative timeout budget on its own definition
rather than a deployment naming it in a central config map. The field
never reaches the model (schemas() whitelists name/description/parameters)
and defineTool rejects a non-positive-finite value at authorship.

packages/core/tools/src/index.ts

@@ -138,6 +138,14 @@ export type ToolExecuteReturn = ContentBlock[] | { content: ContentBlock[]; meta
 /** A registered tool: its schema plus the execution function. */
 export interface ToolDefinition extends ToolSchema {
   execute(args: unknown, exec: ToolExecution): Promise<ToolExecuteReturn>
+  /**
+   * Cooperative tool-call timeout budget in milliseconds. Omit for no deadline.
+   * Enforced by `@deepseek-ai/dsh-timeout-policy` (a `tools/execute` wrapper); it
+   * is NEVER sent to the model — `schemas()` whitelists only name/description/
+   * parameters. Declaring it asserts this tool forwards `exec.signal` to a
+   * cooperative implementation that can reach quiescence when the signal aborts.
+   */
+  timeoutMs?: number
   /**
    * Optional: how to present the PENDING state of one call in a UI, derived from
    * the call's `args` (parsed arguments, `unknown` — the tool validates/narrows

packages/core/tools/src/schema.ts

@@ -295,6 +295,13 @@ export interface DefineToolOptions<S extends SchemaSpec> {
    * standard JSON Schema at runtime.
    */
   parameters: S
+  /**
+   * Optional cooperative tool-call timeout budget in milliseconds. When given it
+   * must be a positive finite number; it is attached to the produced
+   * {@link ToolDefinition} for `@deepseek-ai/dsh-timeout-policy` to enforce and
+   * is never sent to the model.
+   */
+  timeoutMs?: number
   /**
    * Tool execution function. `args` is typed as {@link InferArgs<S>} — zero
    * casts needed. Returns either a bare {@link ContentBlock}`[]` (model-facing
@@ -362,10 +369,14 @@ export function defineTool<S extends SchemaSpec>(options: DefineToolOptions<S>):
   const userPresentCall = options.presentCall
   // eslint-disable-next-line @typescript-eslint/unbound-method
   const userPresentResult = options.presentResult
+  if (options.timeoutMs !== undefined && (!Number.isFinite(options.timeoutMs) || options.timeoutMs <= 0)) {
+    throw new Error(`defineTool(${options.name}): timeoutMs must be a positive finite number`)
+  }
   const tool: ToolDefinition = {
     name: options.name,
     description: options.description,
     parameters: schemaSpecToJsonSchema(options.parameters) as unknown as Record<string, unknown>,
+    ...(options.timeoutMs !== undefined ? { timeoutMs: options.timeoutMs } : {}),
     async execute(args: unknown, exec: ToolExecution): Promise<ToolExecuteReturn> {
       // Validate the model-generated args before the typed body runs. On
       // mismatch we throw ToolArgsError; the registry turns it into an