feat(grep): add ripgrep auto-download and installation

Port ripgrep auto-installation feature from original OpenCode (sst/opencode).
When ripgrep is not available, automatically downloads and installs it from
GitHub releases.

Features:
- Platform detection (darwin/linux/win32, arm64/x64)
- Archive extraction (tar.gz/zip)
- Caches binary in ~/.cache/oh-my-opencode/bin/
- New resolveGrepCliWithAutoInstall() async function
- Falls back to grep if auto-install fails

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/tools/grep/downloader.ts

@@ -0,0 +1,168 @@
+import { existsSync, mkdirSync, chmodSync, unlinkSync } from "node:fs"
+import { join } from "node:path"
+import { spawn } from "bun"
+
+const RG_VERSION = "14.1.1"
+
+const PLATFORM_CONFIG: Record<string, { platform: string; extension: "tar.gz" | "zip" } | undefined> = {
+  "arm64-darwin": { platform: "aarch64-apple-darwin", extension: "tar.gz" },
+  "arm64-linux": { platform: "aarch64-unknown-linux-gnu", extension: "tar.gz" },
+  "x64-darwin": { platform: "x86_64-apple-darwin", extension: "tar.gz" },
+  "x64-linux": { platform: "x86_64-unknown-linux-musl", extension: "tar.gz" },
+  "x64-win32": { platform: "x86_64-pc-windows-msvc", extension: "zip" },
+}
+
+function getPlatformKey(): string {
+  return `${process.arch}-${process.platform}`
+}
+
+function getInstallDir(): string {
+  const homeDir = process.env.HOME || process.env.USERPROFILE || "."
+  return join(homeDir, ".cache", "oh-my-opencode", "bin")
+}
+
+function getRgPath(): string {
+  const isWindows = process.platform === "win32"
+  return join(getInstallDir(), isWindows ? "rg.exe" : "rg")
+}
+
+async function downloadFile(url: string, destPath: string): Promise<void> {
+  const response = await fetch(url)
+  if (!response.ok) {
+    throw new Error(`Failed to download: ${response.status} ${response.statusText}`)
+  }
+
+  const buffer = await response.arrayBuffer()
+  await Bun.write(destPath, buffer)
+}
+
+async function extractTarGz(archivePath: string, destDir: string): Promise<void> {
+  const platformKey = getPlatformKey()
+
+  const args = ["tar", "-xzf", archivePath, "--strip-components=1"]
+
+  if (platformKey.endsWith("-darwin")) {
+    args.push("--include=*/rg")
+  } else if (platformKey.endsWith("-linux")) {
+    args.push("--wildcards", "*/rg")
+  }
+
+  const proc = spawn(args, {
+    cwd: destDir,
+    stdout: "pipe",
+    stderr: "pipe",
+  })
+
+  const exitCode = await proc.exited
+  if (exitCode !== 0) {
+    const stderr = await new Response(proc.stderr).text()
+    throw new Error(`Failed to extract tar.gz: ${stderr}`)
+  }
+}
+
+async function extractZipWindows(archivePath: string, destDir: string): Promise<void> {
+  const proc = spawn(
+    ["powershell", "-Command", `Expand-Archive -Path '${archivePath}' -DestinationPath '${destDir}' -Force`],
+    { stdout: "pipe", stderr: "pipe" }
+  )
+  const exitCode = await proc.exited
+  if (exitCode !== 0) {
+    throw new Error("Failed to extract zip with PowerShell")
+  }
+
+  const { globSync } = await import("glob")
+  const rgFiles = globSync("**/rg.exe", { cwd: destDir })
+  if (rgFiles.length > 0) {
+    const srcPath = join(destDir, rgFiles[0])
+    const destPath = join(destDir, "rg.exe")
+    if (srcPath !== destPath) {
+      const { renameSync } = await import("node:fs")
+      renameSync(srcPath, destPath)
+    }
+  }
+}
+
+async function extractZipUnix(archivePath: string, destDir: string): Promise<void> {
+  const proc = spawn(["unzip", "-o", archivePath, "-d", destDir], {
+    stdout: "pipe",
+    stderr: "pipe",
+  })
+  const exitCode = await proc.exited
+  if (exitCode !== 0) {
+    throw new Error("Failed to extract zip")
+  }
+
+  const { globSync } = await import("glob")
+  const rgFiles = globSync("**/rg", { cwd: destDir })
+  if (rgFiles.length > 0) {
+    const srcPath = join(destDir, rgFiles[0])
+    const destPath = join(destDir, "rg")
+    if (srcPath !== destPath) {
+      const { renameSync } = await import("node:fs")
+      renameSync(srcPath, destPath)
+    }
+  }
+}
+
+async function extractZip(archivePath: string, destDir: string): Promise<void> {
+  if (process.platform === "win32") {
+    await extractZipWindows(archivePath, destDir)
+  } else {
+    await extractZipUnix(archivePath, destDir)
+  }
+}
+
+export async function downloadAndInstallRipgrep(): Promise<string> {
+  const platformKey = getPlatformKey()
+  const config = PLATFORM_CONFIG[platformKey]
+
+  if (!config) {
+    throw new Error(`Unsupported platform: ${platformKey}`)
+  }
+
+  const installDir = getInstallDir()
+  const rgPath = getRgPath()
+
+  if (existsSync(rgPath)) {
+    return rgPath
+  }
+
+  mkdirSync(installDir, { recursive: true })
+
+  const filename = `ripgrep-${RG_VERSION}-${config.platform}.${config.extension}`
+  const url = `https://github.com/BurntSushi/ripgrep/releases/download/${RG_VERSION}/${filename}`
+  const archivePath = join(installDir, filename)
+
+  try {
+    await downloadFile(url, archivePath)
+
+    if (config.extension === "tar.gz") {
+      await extractTarGz(archivePath, installDir)
+    } else {
+      await extractZip(archivePath, installDir)
+    }
+
+    if (process.platform !== "win32") {
+      chmodSync(rgPath, 0o755)
+    }
+
+    if (!existsSync(rgPath)) {
+      throw new Error("ripgrep binary not found after extraction")
+    }
+
+    return rgPath
+  } finally {
+    if (existsSync(archivePath)) {
+      try {
+        unlinkSync(archivePath)
+      } catch {
+        // Cleanup failures are non-critical
+      }
+    }
+  }
+}
+
+export function getInstalledRipgrepPath(): string | null {
+  const rgPath = getRgPath()
+  return existsSync(rgPath) ? rgPath : null
+}