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/index.ts

@@ -1,14 +1,12 @@
-import {
-  ast_grep_search,
-  ast_grep_replace,
-} from "./tools"
+import { ast_grep_search, ast_grep_replace } from "./tools"
 
 export const builtinTools = {
   ast_grep_search,
   ast_grep_replace,
 }
 
-export {
-  ast_grep_search,
-  ast_grep_replace,
-}
+export { ast_grep_search, ast_grep_replace }
+export { ensureAstGrepBinary, getCachedBinaryPath, getCacheDir } from "./downloader"
+export { getAstGrepPath, isCliAvailable, ensureCliAvailable, startBackgroundInit } from "./cli"
+export { checkEnvironment, formatEnvironmentCheck } from "./constants"
+export type { EnvironmentCheckResult } from "./constants"