core: modularize config parsing to improve maintainability

Extract error handling, parsing logic, and variable substitution into dedicated
modules. This reduces duplication between tui.json and opencode.json parsing
and makes the config system easier to extend for future config formats.

packages/opencode/src/config/error.ts

@@ -0,0 +1,21 @@
+export * as ConfigError from "./error"
+
+import z from "zod"
+import { NamedError } from "@opencode-ai/shared/util/error"
+
+export const JsonError = NamedError.create(
+  "ConfigJsonError",
+  z.object({
+    path: z.string(),
+    message: z.string().optional(),
+  }),
+)
+
+export const InvalidError = NamedError.create(
+  "ConfigInvalidError",
+  z.object({
+    path: z.string(),
+    issues: z.custom<z.core.$ZodIssue[]>().optional(),
+    message: z.string().optional(),
+  }),
+)