refactor: clean up verbose and redundant comments

Trim or remove comments that:
- State the obvious (function names already convey purpose)
- Repeat what the code clearly shows
- Are excessively long without adding value

Keep comments that explain *why*, not *what*.

internal/engines/planner.go

@@ -95,9 +95,6 @@ func (p *Planner) Plan(req contracts.SearchRequest) (localEngines, upstreamEngin
 }
 
 func inferFromCategories(categories []string) []string {
-	// Minimal mapping for the initial porting subset.
-	// This mirrors the idea of selecting from engine categories without
-	// embedding the whole engine registry.
 	set := map[string]bool{}
 	for _, c := range categories {
 		switch strings.TrimSpace(strings.ToLower(c)) {
@@ -131,7 +128,6 @@ func inferFromCategories(categories []string) []string {
 }
 
 func sortByOrder(list []string, order map[string]int) {
-	// simple insertion sort (list is tiny)
 	for i := 1; i < len(list); i++ {
 		j := i
 		for j > 0 && order[list[j-1]] > order[list[j]] {