feat(domain): introduce typed error classes and replace fragile string-based error handling

src/domain/errors.ts

@@ -0,0 +1,45 @@
+/**
+ * Base error for all Booster Role bot errors.
+ * Enables reliable error type checking via `instanceof` instead of fragile
+ * string matching on `error.message`.
+ */
+export class BoosterRoleError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "BoosterRoleError";
+  }
+}
+
+/**
+ * Input validation failures — user provided an invalid name, color, icon, etc.
+ * Safe to show the message directly to the end user.
+ */
+export class ValidationError extends BoosterRoleError {
+  constructor(message: string) {
+    super(message);
+    this.name = "ValidationError";
+  }
+}
+
+/**
+ * Resource not found — no stored role, no Discord role, etc.
+ * Safe to show the message directly to the end user.
+ */
+export class NotFoundError extends BoosterRoleError {
+  constructor(message: string) {
+    super(message);
+    this.name = "NotFoundError";
+  }
+}
+
+/**
+ * Permission / authorisation failures — user doesn't own the role, bot
+ * lacks permissions, position is unsafe, etc.
+ * Safe to show the message directly to the end user.
+ */
+export class PermissionError extends BoosterRoleError {
+  constructor(message: string) {
+    super(message);
+    this.name = "PermissionError";
+  }
+}