feat: add read_file tool

utils/instructions.ts

@@ -99,7 +99,7 @@ function getShellInstructions(
     case "restricted":
       return `### Shell commands\n\nUse the \`${t("shell")}\` MCP tool for all shell command execution. This tool provides a secure environment with filtered credentials. Do NOT use any native shell tool — it is disabled for security. For long-running processes, use \`shell({ command, background: true })\`. Use \`${t("kill_background")}\` to stop background processes.`;
     case "enabled":
-      return `### Shell commands\n\nUse the \`${t("shell")}\` MCP tool for all shell command execution (unrestricted environment, credentials available).`;
+      return `### Shell commands\n\nUse your native shell tool for shell command execution.`;
     default: {
       const _exhaustive: never = shell;
       return _exhaustive satisfies never;
@@ -107,11 +107,8 @@ function getShellInstructions(
   }
 }
 
-function getFileInstructions(shell: ResolvedPayload["shell"]): string {
-  if (shell === "disabled") {
-    return `### File operations\n\nShell is disabled — you cannot read arbitrary files. Use the diff content returned by MCP tools directly.`;
-  }
-  return `### File operations\n\nUse the \`shell\` MCP tool for all file operations. Examples:\n- Read a file: \`shell({ command: 'cat /path/to/file', description: 'read file' })\`\n- Read specific lines: \`shell({ command: 'sed -n "<start>,<end>p" /path/to/file', description: 'read lines' })\`\n- Search in a file: \`shell({ command: 'grep -n "pattern" /path/to/file', description: 'search' })\``;
+function getFileInstructions(_shell: ResolvedPayload["shell"]): string {
+  return `### File operations\n\nUse the \`read_file\` MCP tool to read files. For example: \`read_file({ path: diffPath, start_line: 5, end_line: 42 })\`. This is the primary way to read the PR diff returned by \`checkout_pr\`.`;
 }
 
 function getStandaloneModeInstructions(