feat: add PR walkthrough generation workflow (#2933)

.github/scripts/opencode-pr-file-tool.ts

@@ -0,0 +1,41 @@
+import { tool } from "@opencode-ai/plugin"
+import path from "path"
+
+export default tool({
+  description:
+    "Read one regular UTF-8 file from the immutable pull request head. Use this instead of repository read tools for PR-head contents.",
+  args: {
+    path: tool.schema
+      .string()
+      .max(4096)
+      .describe("Strict repository-relative path at the pull request head"),
+  },
+  async execute(args, context) {
+    const helper = path.join(context.worktree, ".opencode-walkthrough", "read-file")
+    const sourceEnv = globalThis.process.env
+    const headSha = sourceEnv.HEAD_SHA ?? ""
+    const childEnv = Object.fromEntries(
+      ["PATH"]
+        .filter((name) => sourceEnv[name] !== undefined)
+        .map((name) => [name, sourceEnv[name] as string]),
+    )
+    const child = Bun.spawn(
+      ["python3", helper, "--repo", context.worktree, "--head-sha", headSha, "--path", args.path],
+      {
+        cwd: context.worktree,
+        env: childEnv,
+        stdout: "pipe",
+        stderr: "pipe",
+      },
+    )
+    const [status, stdout, stderr] = await Promise.all([
+      child.exited,
+      new Response(child.stdout).text(),
+      new Response(child.stderr).text(),
+    ])
+    if (status !== 0) {
+      return `PR file read rejected. Choose a regular UTF-8 file from the changed-file list.\n${stderr || stdout}`
+    }
+    return stdout
+  },
+})