feat(agents): add file:// URI support in prompt_append configuration

Port devxoul's PR #821 feature to current codebase structure.
Supports absolute, relative, ~/home paths with percent-encoding.
Gracefully handles malformed URIs and missing files with warnings.

Co-authored-by: devxoul <devxoul@gmail.com>

src/agents/builtin-agents/resolve-file-uri.ts

@@ -0,0 +1,30 @@
+import { existsSync, readFileSync } from "node:fs"
+import { homedir } from "node:os"
+import { isAbsolute, resolve } from "node:path"
+
+export function resolvePromptAppend(promptAppend: string, configDir?: string): string {
+  if (!promptAppend.startsWith("file://")) return promptAppend
+
+  const encoded = promptAppend.slice(7)
+
+  let filePath: string
+  try {
+    const decoded = decodeURIComponent(encoded)
+    const expanded = decoded.startsWith("~/") ? decoded.replace(/^~\//, `${homedir()}/`) : decoded
+    filePath = isAbsolute(expanded)
+      ? expanded
+      : resolve(configDir ?? process.cwd(), expanded)
+  } catch {
+    return `[WARNING: Malformed file URI (invalid percent-encoding): ${promptAppend}]`
+  }
+
+  if (!existsSync(filePath)) {
+    return `[WARNING: Could not resolve file URI: ${promptAppend}]`
+  }
+
+  try {
+    return readFileSync(filePath, "utf8")
+  } catch {
+    return `[WARNING: Could not read file: ${promptAppend}]`
+  }
+}