refactor: unify tool reason requirements

.opencode/tool-reason.ts

@@ -0,0 +1,10 @@
+import { tool } from "./tool.js";
+
+// One field and format for optional purpose and required business justification.
+export const toolReason = tool.schema.string().trim().min(1).max(2000);
+export const optionalToolReason = toolReason.optional().describe(
+  "Optional reason: one concise sentence explaining the specific purpose. Omit when the command already makes it clear.",
+);
+export const learningToolReason = toolReason.optional().describe(
+  "Reason: one concise sentence explaining the durable value or change basis. Required for every write, update or removal; optional for list.",
+);