fix(util): rune-safe truncation across formatters

firstLineOf (query) and truncateString (edit) sliced strings at a raw byte
offset, splitting a multibyte UTF-8 character into invalid bytes in
LLM-facing output (ast_query compact format and edit not-found errors are
hot paths). Extract the rune-boundary backoff already used by search's
truncateLine into util.RuneBoundary and route all three sites through it.

Adds table-driven tests for the primitive and a multibyte case for firstLineOf.

internal/query/query.go

@@ -583,7 +583,7 @@ func firstLineOf(s string, maxLen int) string {
 			continue
 		}
 		if len(line) > maxLen {
-			return line[:maxLen-1] + "…"
+			return line[:util.RuneBoundary(line, maxLen-1)] + "…"
 		}
 		return line
 	}