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/variable.ts

@@ -0,0 +1,84 @@
+export * as ConfigVariable from "./variable"
+
+import path from "path"
+import os from "os"
+import { Filesystem } from "@/util"
+import { InvalidError } from "./error"
+
+type ParseSource =
+  | {
+      type: "path"
+      path: string
+    }
+  | {
+      type: "virtual"
+      source: string
+      dir: string
+    }
+
+function source(input: ParseSource) {
+  return input.type === "path" ? input.path : input.source
+}
+
+function dir(input: ParseSource) {
+  return input.type === "path" ? path.dirname(input.path) : input.dir
+}
+
+/** Apply {env:VAR} and {file:path} substitutions to config text. */
+export async function substitute(text: string, input: ParseSource, missing: "error" | "empty" = "error") {
+  text = text.replace(/\{env:([^}]+)\}/g, (_, varName) => {
+    return process.env[varName] || ""
+  })
+
+  const fileMatches = Array.from(text.matchAll(/\{file:[^}]+\}/g))
+  if (!fileMatches.length) return text
+
+  const configDir = dir(input)
+  const configSource = source(input)
+  let out = ""
+  let cursor = 0
+
+  for (const match of fileMatches) {
+    const token = match[0]
+    const index = match.index!
+    out += text.slice(cursor, index)
+
+    const lineStart = text.lastIndexOf("\n", index - 1) + 1
+    const prefix = text.slice(lineStart, index).trimStart()
+    if (prefix.startsWith("//")) {
+      out += token
+      cursor = index + token.length
+      continue
+    }
+
+    let filePath = token.replace(/^\{file:/, "").replace(/\}$/, "")
+    if (filePath.startsWith("~/")) {
+      filePath = path.join(os.homedir(), filePath.slice(2))
+    }
+
+    const resolvedPath = path.isAbsolute(filePath) ? filePath : path.resolve(configDir, filePath)
+    const fileContent = (
+      await Filesystem.readText(resolvedPath).catch((error: NodeJS.ErrnoException) => {
+        if (missing === "empty") return ""
+
+        const errMsg = `bad file reference: "${token}"`
+        if (error.code === "ENOENT") {
+          throw new InvalidError(
+            {
+              path: configSource,
+              message: errMsg + ` ${resolvedPath} does not exist`,
+            },
+            { cause: error },
+          )
+        }
+        throw new InvalidError({ path: configSource, message: errMsg }, { cause: error })
+      })
+    ).trim()
+
+    out += JSON.stringify(fileContent).slice(1, -1)
+    cursor = index + token.length
+  }
+
+  out += text.slice(cursor)
+  return out
+}