fix(ast_query): skip dependency dirs (node_modules/vendor) by default

ast_query walked every language-matching file under the search root, including
node_modules, vendor and bower_components — parsing third-party code and
flooding results with irrelevant matches (large token cost on every call,
worst for JS/TS node_modules). Skip these by name during the walk, alongside
the existing hidden-dir skip.

The walk root is now exempt from both skips, so targeting a dependency dir
explicitly (paths:["vendor/foo"]) still works — previously naming a hidden or
dependency dir as the root would have skipped everything. Documents the scope
in the tool description and help resource.

Adds a regression test covering default-skip and explicit-target inclusion.

internal/server/handlers_ast.go

@@ -131,7 +131,10 @@ func (s *Server) runASTQueryWalk(ctx context.Context, p *astQueryParams, exts []
 				return nil
 			}
 			if info.IsDir() {
-				if strings.HasPrefix(info.Name(), ".") {
+				// Skip hidden and dependency dirs (third-party code that would
+				// flood results), but never the walk root itself — the caller
+				// explicitly targeted it, e.g. paths:["vendor/foo"].
+				if path != searchPath && (strings.HasPrefix(info.Name(), ".") || isDependencyDir(info.Name())) {
 					return filepath.SkipDir
 				}
 				return nil
@@ -172,6 +175,19 @@ func (s *Server) runASTQueryWalk(ctx context.Context, p *astQueryParams, exts []
 	return allResults
 }
 
+// isDependencyDir reports whether name is a well-known third-party dependency
+// directory that ast_query skips by default. These hold vendored/installed code,
+// not the caller's source, and parsing them wastes time and floods results with
+// irrelevant matches. Target one explicitly via paths to query inside it.
+func isDependencyDir(name string) bool {
+	switch name {
+	case "node_modules", "vendor", "bower_components":
+		return true
+	default:
+		return false
+	}
+}
+
 // hasAnySuffix reports whether path ends with any of the given suffixes.
 func hasAnySuffix(path string, suffixes []string) bool {
 	for _, ext := range suffixes {