feat: add read_file tool

mcp/readFile.ts

@@ -0,0 +1,38 @@
+import { readFileSync } from "node:fs";
+import { type } from "arktype";
+import type { ToolContext } from "./server.ts";
+import { execute, tool } from "./shared.ts";
+
+export const ReadFileParams = type({
+  path: type.string.describe("absolute path to the file to read"),
+  "start_line?": type.number.describe("start line, 1-based inclusive (default: 1)"),
+  "end_line?": type.number.describe("end line, 1-based inclusive (default: end of file)"),
+});
+
+export function ReadFileTool(_ctx: ToolContext) {
+  return tool({
+    name: "read_file",
+    description:
+      "Read lines from a file. Use this to read sections of the PR diff returned by checkout_pr. " +
+      "Example: `read_file({ path: diffPath, start_line: 5, end_line: 42 })`.",
+    parameters: ReadFileParams,
+    execute: execute(async ({ path, start_line, end_line }) => {
+      let content: string;
+      try {
+        content = readFileSync(path, "utf-8");
+      } catch (err) {
+        const msg = err instanceof Error ? err.message : String(err);
+        throw new Error(`failed to read ${path}: ${msg}`);
+      }
+
+      if (start_line === undefined && end_line === undefined) {
+        return { content };
+      }
+
+      const lines = content.split("\n");
+      const start = Math.max(0, (start_line ?? 1) - 1);
+      const end = Math.min(lines.length, end_line ?? lines.length);
+      return { content: lines.slice(start, end).join("\n") };
+    }),
+  });
+}