feat(ast-grep): add CLI path resolution and auto-download functionality

- Add automatic CLI binary path detection and resolution
- Implement lazy binary download with caching
- Add environment check utilities for CLI and NAPI availability
- Improve error handling and fallback mechanisms
- Export new utilities from index.ts

src/tools/ast-grep/downloader.ts

@@ -0,0 +1,151 @@
+import { spawn } from "bun"
+import { existsSync, mkdirSync, chmodSync, unlinkSync } from "fs"
+import { join } from "path"
+import { homedir } from "os"
+import { createRequire } from "module"
+
+const REPO = "ast-grep/ast-grep"
+
+// IMPORTANT: Update this when bumping @ast-grep/cli in package.json
+// This is only used as fallback when @ast-grep/cli package.json cannot be read
+const DEFAULT_VERSION = "0.40.0"
+
+function getAstGrepVersion(): string {
+  try {
+    const require = createRequire(import.meta.url)
+    const pkg = require("@ast-grep/cli/package.json")
+    return pkg.version
+  } catch {
+    return DEFAULT_VERSION
+  }
+}
+
+interface PlatformInfo {
+  arch: string
+  os: string
+}
+
+const PLATFORM_MAP: Record<string, PlatformInfo> = {
+  "darwin-arm64": { arch: "aarch64", os: "apple-darwin" },
+  "darwin-x64": { arch: "x86_64", os: "apple-darwin" },
+  "linux-arm64": { arch: "aarch64", os: "unknown-linux-gnu" },
+  "linux-x64": { arch: "x86_64", os: "unknown-linux-gnu" },
+  "win32-x64": { arch: "x86_64", os: "pc-windows-msvc" },
+  "win32-arm64": { arch: "aarch64", os: "pc-windows-msvc" },
+  "win32-ia32": { arch: "i686", os: "pc-windows-msvc" },
+}
+
+export function getCacheDir(): string {
+  if (process.platform === "win32") {
+    const localAppData = process.env.LOCALAPPDATA || process.env.APPDATA
+    const base = localAppData || join(homedir(), "AppData", "Local")
+    return join(base, "oh-my-opencode", "bin")
+  }
+
+  const xdgCache = process.env.XDG_CACHE_HOME
+  const base = xdgCache || join(homedir(), ".cache")
+  return join(base, "oh-my-opencode", "bin")
+}
+
+export function getBinaryName(): string {
+  return process.platform === "win32" ? "sg.exe" : "sg"
+}
+
+export function getCachedBinaryPath(): string | null {
+  const binaryPath = join(getCacheDir(), getBinaryName())
+  return existsSync(binaryPath) ? binaryPath : null
+}
+
+async function extractZip(archivePath: string, destDir: string): Promise<void> {
+  const proc =
+    process.platform === "win32"
+      ? spawn(
+          [
+            "powershell",
+            "-command",
+            `Expand-Archive -Path '${archivePath}' -DestinationPath '${destDir}' -Force`,
+          ],
+          { stdout: "pipe", stderr: "pipe" }
+        )
+      : spawn(["unzip", "-o", archivePath, "-d", destDir], { stdout: "pipe", stderr: "pipe" })
+
+  const exitCode = await proc.exited
+
+  if (exitCode !== 0) {
+    const stderr = await new Response(proc.stderr).text()
+    const toolHint =
+      process.platform === "win32"
+        ? "Ensure PowerShell is available on your system."
+        : "Please install 'unzip' (e.g., apt install unzip, brew install unzip)."
+    throw new Error(`zip extraction failed (exit ${exitCode}): ${stderr}\n\n${toolHint}`)
+  }
+}
+
+export async function downloadAstGrep(version: string = DEFAULT_VERSION): Promise<string | null> {
+  const platformKey = `${process.platform}-${process.arch}`
+  const platformInfo = PLATFORM_MAP[platformKey]
+
+  if (!platformInfo) {
+    console.error(`[oh-my-opencode] Unsupported platform for ast-grep: ${platformKey}`)
+    return null
+  }
+
+  const cacheDir = getCacheDir()
+  const binaryName = getBinaryName()
+  const binaryPath = join(cacheDir, binaryName)
+
+  if (existsSync(binaryPath)) {
+    return binaryPath
+  }
+
+  const { arch, os } = platformInfo
+  const assetName = `app-${arch}-${os}.zip`
+  const downloadUrl = `https://github.com/${REPO}/releases/download/${version}/${assetName}`
+
+  console.log(`[oh-my-opencode] Downloading ast-grep binary...`)
+
+  try {
+    if (!existsSync(cacheDir)) {
+      mkdirSync(cacheDir, { recursive: true })
+    }
+
+    const response = await fetch(downloadUrl, { redirect: "follow" })
+
+    if (!response.ok) {
+      throw new Error(`HTTP ${response.status}: ${response.statusText}`)
+    }
+
+    const archivePath = join(cacheDir, assetName)
+    const arrayBuffer = await response.arrayBuffer()
+    await Bun.write(archivePath, arrayBuffer)
+
+    await extractZip(archivePath, cacheDir)
+
+    if (existsSync(archivePath)) {
+      unlinkSync(archivePath)
+    }
+
+    if (process.platform !== "win32" && existsSync(binaryPath)) {
+      chmodSync(binaryPath, 0o755)
+    }
+
+    console.log(`[oh-my-opencode] ast-grep binary ready.`)
+
+    return binaryPath
+  } catch (err) {
+    console.error(
+      `[oh-my-opencode] Failed to download ast-grep: ${err instanceof Error ? err.message : err}`
+    )
+    return null
+  }
+}
+
+export async function ensureAstGrepBinary(): Promise<string | null> {
+  const cachedPath = getCachedBinaryPath()
+  if (cachedPath) {
+    return cachedPath
+  }
+
+  const version = getAstGrepVersion()
+  return downloadAstGrep(version)
+}